Fix generation of BIDS changelog

[sappelhoff]

Replaces #1835, so that this is a PR not from a fork, but from the org itself.

This is an attempt to work around the "Pass secrets to builds from forked pull requests" CircleCI setting, which we'd rather leave "switched off" for security reasons.

EDIT: will migrate this to GH actions, as discussed in maintainers meeting.

NOTE: authentification is working, but we run into secondary API limits, so the workflow may take a long time to run!

... mentioned in #1844

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.04%. Comparing base (124ac21) to head (2eb872e).
Report is 256 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1836   +/-   ##
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  Coverage   88.04%   88.04%           
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  Files          16       16           
  Lines        1380     1380           
=======================================
  Hits         1215     1215           
  Misses        165      165           

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[sappelhoff]

Okay, I am confident that this PR now fixes our changelog situation. I have tested the last stage (after a merge commit) also in https://github.com/sappelhoff/testing and everything runs as expected.

All in all, this PR:

1. moves the changelog generation from circleci to GH actions
2. updates the changelog generation software to a newer version
3. fixes the ensuing bugs that were previously uncaught

with this in place, we should have time to tackle a migration of the changelog generation as discussed in:

• Change the way the BIDS spec changelog is created. #1844

... without stressing over it.