Fix #19507 Prevent final reassignment in match case

[omerhadari]

Fixes #19507

This PR adds a check to prevent reassignment of final variables in a match statement.

Currently, the following passes without an error:

from typing import Final
FOO: Final = 8

a = 10

match a:
    case FOO:
        pass
print(FOO)  # FOO is reassigned, prints 10

MyPy already checks that the type of FOO isn't changed if used like this. I added a check in the same place that makes sure it's not Final either.

Since this tests the match syntax, I put the test where I did. If it's not the appropriate place for it I will move it as instructed :)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Great, thanks!

[hauntsaninja]

Thank you!