Constrain memoryview type var to allowed types

[srittau]

No description provided.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

I haven't fully thought this through but #13519 partially made me think that making memoryview generic with a default was a mistake. memoryview now means memoryview[int], so if you have a memoryview[bytes] you can't pass it to a function that wants a plain memoryview. But usually that's wrong; lots of memoryview functionality doesn't depend on the type parameter and lots of functions will work with any kind of memoryview.

[srittau]

I was thinking about that, but I think changing the default to Any might be a better solution. That keeps the flexibility of restricting memoryviews while by default any memoryview is accepted.