BF: update changelog conversion from .md to .rst (for sphinx)

[yarikoptic]

Seems to need quite a bit of workarounds to get an .rst version. See diff comment in Makefile -- sphinx isn't that ready for unicode in section headings, eh heh

TODOs

• @jwodder -- we need to call make update-changelog (requires iconv and pandoc) after a new entry for CHANGELOG.md is generated but before it is committed. Adjustments to the changelog.rst in docs/ should be committed as well. If you see how to improve that ugly Makefile rule , feel welcome ;)
• added allowing for "tests" label without patch or minor

[codecov]

Codecov Report

Merging #5757 (46f2327) into maint (985413a) will decrease coverage by 36.66%.
The diff coverage is 55.55%.

❗ Current head 46f2327 differs from pull request most recent head 075e962. Consider uploading reports for the commit 075e962 to get more accurate results

@@             Coverage Diff             @@
##            maint    #5757       +/-   ##
===========================================
- Coverage   89.89%   53.22%   -36.67%     
===========================================
  Files         299      297        -2     
  Lines       42350    42302       -48     
===========================================
- Hits        38069    22515    -15554     
- Misses       4281    19787    +15506     

Impacted Files	Coverage Δ
datalad/core/distributed/clone.py	89.84% <ø> (-0.26%)	⬇️
datalad/core/distributed/push.py	88.52% <ø> (ø)
datalad/core/local/create.py	92.36% <ø> (-0.70%)	⬇️
datalad/core/local/diff.py	90.62% <ø> (ø)
datalad/core/local/run.py	98.23% <ø> (-0.89%)	⬇️
datalad/core/local/status.py	96.42% <ø> (ø)
datalad/distributed/create_sibling_ria.py	59.41% <ø> (-22.95%)	⬇️
datalad/distributed/export_archive_ora.py	38.46% <ø> (-43.08%)	⬇️
datalad/distribution/create_sibling.py	47.60% <ø> (-38.31%)	⬇️
datalad/distribution/create_sibling_github.py	84.37% <ø> (ø)
... and 242 more

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[jwodder]

@yarikoptic Rather than futzing with pull requests on pull requests, I'll just tell you what changes need to be made.

• Modify .github/workflows/release.yml to install pandoc before running ~/auto shipit (iconv is already installed)

• Add the following line to .autorc above the "afterRelease" line:

    "afterChangelog": "make update-changelog && git add docs/source/changelog.rst && git commit -m '[skip ci] Update RST changelog'",
  

  It might be cleaner to add the git add && git commit to the Makefile rule instead, though.

Also, note the following:

• Assuming that the Unicode problem is entirely due to the emojis in the label sections, an alternative to using iconv would be to modify the labels' changelogTitle strings in the configuration.
• If we ever run into the E2BIG error again, I believe it would now occur at the afterChangelog stage, killing the release entirely. The best way around this (as far as I know) would be this plugin, but the devs haven't responded to my comment about how exactly to use it. I'm going to try it out in a test repository and get back to you.

[yarikoptic]

Thank you @jwodder !!! I will meanwhile merge as is, since we might need to kick out release soonish to accompany joss publication.