Add GitPython to "docs" optional deps

[khaeru]

This was missed in #719. It is required during the Sphinx docs build, in order for the ixmp.utils.sphinx_linkcode_github extension to identify the branch name to link, in both:

• RTD (example failed build here).
• The "pytest" CI workflow run on a schedule event (example failed job here).

This issue was/is not visible on the CI workflow job on the PR branch, because in that case the explicitly supplied head_ref is used:

message_ix/.github/workflows/pytest.yaml

Lines 127 to 131 in 4034d8c

	env:
	# For pull_request triggers, GitHub creates a temporary merge commit
	# with a hash that does not match the head of the branch. Tell it which
	# branch to use.
	SPHINXOPTS: -D linkcode_github_remote_head=${{ github.head_ref }}

The added dependency mirrors ixmp; in the ixmp CI the extension is working properly.

How to review

See the log of this example build from the current branch. In the final step, the 4th line of output is:

linkcode base URL: https://github.com/iiasa/message_ix/blob/fix/docs-ci

This indicates the branch is correctly identified using GitPython.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A
• Update release notes.

[glatterf42]

Thanks, I was about to do the same ;)

[glatterf42]

This should pass the tests after merging, so it looks good to me.

[codecov]

Codecov Report

Merging #729 (2e0a9c4) into main (4034d8c) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #729   +/-   ##
=====================================
  Coverage   94.4%   94.4%           
=====================================
  Files         43      43           
  Lines       3434    3434           
=====================================
+ Hits        3244    3245    +1     
+ Misses       190     189    -1     

see 1 file with indirect coverage changes