chore(deps): add pygit to doc requirements

[mr-cal]

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• Have you successfully run pytest tests/unit?

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#4368 broke sphinx:

 python -m sphinx -T -E -W --keep-going -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html
Running Sphinx v7.2.6
making output directory... done
loading intersphinx inventory from https://docs.python.org/3/objects.inv...
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/gen_cli_docs.py", line 13, in <module>
    from snapcraft import cli
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/../../snapcraft/cli.py", line 37, in <module>
    from . import commands
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/../../snapcraft/commands/__init__.py", line 63, in <module>
    from .remote import RemoteBuildCommand
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/../../snapcraft/commands/remote.py", line 33, in <module>
    from snapcraft.remote import is_repo
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/../../snapcraft/remote/__init__.py", line 20, in <module>
    from .git import GitRepo, is_repo
  File "/home/docs/checkouts/readthedocs.org/user_builds/canonical-snapcraft/checkouts/latest/tools/docs/../../snapcraft/remote/git.py", line 22, in <module>
    import pygit2
ModuleNotFoundError: No module named 'pygit2'