Warn users on `~=` python version specifier (#14008)
Close #7426
## Summary
Picking up on #8284, I noticed that the `requires_python` object already
has its specifiers canonicalized in the `intersection` method, meaning
`~=3.12` is converted to `>=3.12, <4`. To fix this, we check and warn in
`intersection`.
## Test Plan
Used the same tests from #8284.
