Modify Repository.get_contents() to always return List[github.ContentFile.ContentFile]

[pjames5]

Hello!

I made some recent updates to application that uses Repository.get_contents() and ran into some typing issues with a recent mypy update.

My proposal is to modify the return type of Repository.get_contents() to be List[github.ContentFile.ContentFile] instead of Union[github.ContentFile.ContentFile, List[github.ContentFile.ContentFile], None]. This would help to clean up the mypy issues and make the API clearer.

Something like this:

        if isinstance(data, list):
            return [
                # Lazy completion only makes sense for files. See discussion
                # here: https://github.com/jacquev6/PyGithub/issues/140#issuecomment-13481130
                github.ContentFile.ContentFile(
                    self._requester, headers, item, completed=(item["type"] != "file")
                )
                for item in data
            ]
        return [
            github.ContentFile.ContentFile(
                self._requester, headers, data, completed=True
            )
        ]

I'm happy to open a PR with this change or discuss further. Thanks!