With GitPython==3.1.42 one can observe following annotation of parent_commits argument to IndexFile.commit ([1]):

...
parent_commits: Union[Commit_ish, None] = None,
...

which is obviously not correct, because that argument just passed unmodified downwards to Commit.create_from_tree, which has following annotation instead ([2]):

parent_commits: Union[None, List["Commit"]] = None

That leads to nasty Mypy warnings when one's trying to use IndexFile.commit with anything different than None.

[1] - https://github.com/gitpython-developers/GitPython/blob/3.1.42/git/index/base.py#L1080
[2] - https://github.com/gitpython-developers/GitPython/blob/3.1.42/git/objects/commit.py#L506