Use @overload to type `get_user` (#3502)

LSPs and type checkers complain about
`Github(...).get_user().create_repo(...)` and other such calls which
only work on the authenticated user.

Because whether the user is named or authenticated depends entirely on a
`login` being provided or not, `overload` allows statically separating
these two cases and makes typecheckers happier.

Demo on the mypy playground:
- [current
(union)](https://mypy-play.net/?gist=6f5eb7865f4549ec041b8df3fee14805)
- [proposal
(overload)](https://mypy-play.net/?gist=40490f9b0f58b40b3f60ca908583de24)

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>