<eschwartz> warning: I just released calibre 5.0 to the arch repos. It's python3-only, so I dropped some old modules I don't need anymore.
<eschwartz> You do need them, for calibre 3.x sans webengine -- so before the next parabola sync, you need to add them yourself
<eschwartz> you can see the PKGBUILDs I modified/dropped here: https://git.archlinux.org/svntogit/community.git/log/?qt=grep&q=python2
<eschwartz> basically you need python2-{apsw,css-parser,html2text,html5-parser,markdown,mechanize,netifaces,pychm,regex,unrardll} - possibly not pychm
<eschwartz> you'll need to also add some pyqt stuff whenever arojas drops those from [extra]

the dependency chain in the OP was based on the deps for calibre v4 - for calibre3 these 8 are unavailable:

$ pkgs=$(pactree ${OPTS} --sync calibre3) # where $OPTS is a custom --dbpath= --config=
                                          # searching all repos
$ for pkg in $pkgs ; do pacman -Ssq $pkg | grep ^$pkg$ > /dev/null || echo "n/a: $pkg" ; done
n/a: python2-apsw
n/a: python2-css-parser
n/a: python2-html2text
n/a: python2-html5-parser
n/a: python2-mechanize
n/a: python2-netifaces
n/a: python2-pyqt5
n/a: python2-regex

re: python2-unrardll (mentioned by eli) - that is the original reason why calibre is on the blacklist (no unrar)

now that arch has dropped python2, this is the final list of affected parabola packages

$ sudo parabola-dependents -v python2
updating database ....
package: 'python2' not found - creating dummy package
dummy package 'python2' created
querying database ....
compiling results for (143) dependents ....
searching abslibre ....

direct and transitive parabola dependents:
  [python2]  <- pcr/apenwarr-redo
  [python2]  <- libre/asciidoc
  [python2]  <- libre/b43-tools
  [python2]  <- pcr/boswars
  [python2]  <- pcr/cherokee
  [python2]  <- pcr/cwiid
  [python2]  <- pcr/darcsver
  [python2]  <- pcr/django-social-auth
  [python2]  <- pcr/dratmenu
  [python2]  <- pcr/ganglia
  [python2]  <- pcr/gtg
  [python2]  <- pcr/h-client
  [python2]  <- pcr/idjc
  [python2]  <- pcr/ipycli-git
  [python2]  <- pcr/kamailio-python-modules
  [python2]  <- libre/kodi-bin
  [python2]  <- libre/kodi-gbm
  [python2]  <- kodi-gbm <- libre/kodi
  [python2]  <- kodi-gbm <- kodi <- libre/kodi-dev
  [python2]  <- kodi-gbm <- kodi <- pcr/kodi-standalone-service
  [python2]  <- kodi-gbm <- kodi <- libre/kodi-wayland
  [python2]  <- kodi-gbm <- kodi <- libre/kodi-x11
  [python2]  <- pcr/mimms
  [python2]  <- pcr/mkbootimg
  [python2]  <- pcr/mozilla-addon-sdk
  [python2]  <- pcr/mozilla-devscripts
  [python2]  <- pcr/netatalk
  [python2]  <- pcr/nevow
  [python2]  <- nevow <- pcr/tahoe-lafs
  [python2]  <- pcr/ocrodjvu
  [python2]  <- pcr/openchange
  [python2]  <- openchange <- pcr/evolution-mapi
  [python2]  <- libre/parabolaweb-utils
  [python2]  <- pcr/phantomjs
  [python2]  <- pcr/pycryptopp
  [python2]  <- pcr/pysheng
  [python2]  <- pcr/python2-arxiv2bib
  [python2]  <- pcr/python2-async
  [python2]  <- pcr/python2-authres
  [python2]  <- libre/python2-bencode
  [python2]  <- pcr/python2-bibtexparser
  [python2]  <- pcr/python2-caldav
  [python2]  <- python2-caldav <- pcr/gnuhealth
  [python2]  <- pcr/python2-carrot
  [python2]  <- libre/python2-django1.8
  [python2]  <- python2-django1.8 <- libre/python2-django-countries
  [python2]  <- python2-django1.8 <- libre/python2-django-jinja
  [python2]  <- python2-django1.8 <- pcr/python2-django-tagging
  [python2]  <- pcr/python2-djvulibre
  [python2]  <- pcr/python2-dukpy
  [python2]  <- python2-dukpy <- libre/calibre3
  [python2]  <- pcr/python2-filetype
  [python2]  <- pcr/python2-flask-lesscss
  [python2]  <- pcr/python2-habanero
  [python2]  <- pcr/python2-interruptingcow
  [python2]  <- pcr/python2-isbnlib
  [python2]  <- libre/python2-jsmin
  [python2]  <- pcr/python2-liblarch
  [python2]  <- libre/python2-markdown2
  [python2]  <- libre/python2-markupsafe0
  [python2]  <- python2-markupsafe0 <- pcr/mailpile
  [python2]  <- pcr/python2-paste-deploy
  [python2]  <- python2-paste-deploy <- pcr/python2-paste-script
  [python2]  <- python2-paste-deploy <- python2-paste-script <- pcr/python2-pylons
  [python2]  <- pcr/python2-pefile
  [python2]  <- libre/python2-pgpdump
  [python2]  <- pcr/python2-pkgtools
  [python2]  <- pcr/python2-polib
  [python2]  <- python2-polib <- pcr/trytond
  [python2]  <- python2-polib <- trytond <- pcr/trytond-account
  [python2]  <- python2-polib <- trytond <- trytond-account <- pcr/trytond-account-invoice
  [python2]  <- python2-polib <- trytond <- trytond-account <- trytond-account-invoice <- pcr/trytond-account-invoice-stock
  [python2]  <- python2-polib <- trytond <- trytond-account <- trytond-account-invoice <- trytond-account-invoice-stock <- pcr/trytond-purchase
  [python2]  <- python2-polib <- trytond <- trytond-account <- trytond-account-invoice <- trytond-account-invoice-stock <- trytond-purchase <- pcr/trytond-stock-supply
  [python2]  <- python2-polib <- trytond <- trytond-account <- pcr/trytond-account-product
  [python2]  <- python2-polib <- trytond <- pcr/trytond-calendar
  [python2]  <- python2-polib <- trytond <- pcr/trytond-company
  [python2]  <- python2-polib <- trytond <- trytond-company <- pcr/trytond-stock
  [python2]  <- python2-polib <- trytond <- trytond-company <- trytond-stock <- pcr/trytond-stock-lot
  [python2]  <- python2-polib <- trytond <- pcr/trytond-country
  [python2]  <- python2-polib <- trytond <- trytond-country <- pcr/trytond-party
  [python2]  <- python2-polib <- trytond <- pcr/trytond-currency
  [python2]  <- python2-polib <- trytond <- pcr/trytond-product
  [python2]  <- python2-polib <- trytond <- pcr/trytond-webdav
  [python2]  <- pcr/python2-potr
  [python2]  <- libre/python2-psycopg2.6
  [python2]  <- pcr/python2-pycrypto
  [python2]  <- pcr/python2-pyliblzma
  [python2]  <- pcr/python2-pyparser
  [python2]  <- pcr/python2-pyutil
  [python2]  <- python2-pyutil <- pcr/python2-zbase32
  [python2]  <- python2-pyutil <- pcr/zfec
  [python2]  <- pcr/python2-pywebdav
  [python2]  <- pcr/python2-pywebdav3
  [python2]  <- pcr/python2-qrtools
  [python2]  <- libre/python2-reportlab
  [python2]  <- pcr/python2-rospkg
  [python2]  <- pcr/python2-slugify
  [python2]  <- pcr/python2-sql
  [python2]  <- pcr/python2-stdnum
  [python2]  <- pcr/python2-vcstools
  [python2]  <- pcr/python2-weberror
  [python2]  <- pcr/python2-webhelpers
  [python2]  <- pcr/python2-wsgiref
  [python2]  <- pcr/pyzenity
  [python2]  <- pcr/rencode
  [python2]  <- pcr/textext
  [python2]  <- pcr/timekpr
  [python2]  <- pcr/youtube-dl-gui
  [python2]  <- pcr/zeronet-git

parabola build dependents:
  pcr/beecrypt
  pcr/triceratops-lv2
  pcr/kodi-standalone-service

In #3237 I made a table of packages which can be purged without controversy.

As for almost all of these other packages, I'd also recommend purging.
Python 2 went EOL about 2.75 years ago, so the maintainers of these packages have been given a more than generous time window to port them to Python 3, rewrite them in another programming language, or otherwise remove the dependence on Python 2.
There is no reason to expect those maintainers to suddenly start working on them now.

If users find that a package they use is gone or broken due to the removal of Python 2, they will surely report the issue and we can rescue that package if the maintenance burden vs desirability tradeoff is in the favour of doing so.
Nothing in that list really jumps out to me as desireable other than the few packages in the libre repo we need to keep in order to remain comaptible with Arch.
For AsciiDoc and Kodi, I think Python 2 is just a scripting interface, so they should be okay to keep with minimal modifications.

For purged packages written in an unmaintained scripting language, I say good riddance.

the maintainers of these packages have been given a more than generous time

it is a problem for some libre replacements such as calibre -
the calibre devs did migrate it to python3; but they also added
webengine; so the python3 version of calibre is useless to
parabola - it is a quite a popular program; but it probably just
needs to go away now