Allow None vs TypeVar overlap for overloads (#15846)
Fixes #8881
This is technically unsafe, and I remember we explicitly discussed this
a while ago, but related use cases turn out to be more common than I
expected (judging by how popular the issue is). Also the fix is really
simple.
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Co-authored-by: Ivan Levkivskyi <ilevkivskyi@hopper.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
