Improvements to descriptor `__set__`

[bzoracler]

1. Fix wrong error location when assignment errors is due to __set__. Presently, the error location appears on the right:

   # mypy: pretty=True
   
   import typing as t
   
   class desc:
       __set__ = None
   
   class A:
       prop = desc()
   

   >>> abc: A = A()
   >>> abc.prop = 123
                  ^~~
                  E
   

   https://github.com/python/mypy/commit/e570aa1cd8b964a7c32b8a6d1bcefd8d8da4d4db will change the error location to be on the left:

   >>> abc: A = A()
   >>> abc.prop = 123
       ^~~~~~~~
       E
   

2. Fixes #14969

   • The example in the issue will still error because ClassVar there is not specified with a type (an unannotated ClassVar is equivalent to ClassVar[Any]).
   • Unlike in other annotation contexts, I didn't think it was a good idea to be permissive with Any, due to the potential creep of false negatives.

[github-actions[bot]]

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

[github-actions[bot]]

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