Use more descriptive type than _BaseVersion #9567
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Using _BaseVersion does not allow mypy to know about the Version methods
and properties. For example: .is_prerelease. By using the more
descriptive type, mypy is able to know about these and use them to
verify types.
This is the approach taken by the upstream packaging project. See:
https://github.com/pypa/packaging/blob/20.9/packaging/version.py
Beyond the internal benefit, this will help downstream projects such as
pip-tools which use pip with mypy:
https://github.com/jazzband/pip-tools/blob/5.5.0/piptools/exceptions.py#L20
Currently, the above fails mypy checks with the error: