Duplicate fields in Enum must raise an error

[sobolevn]

I have this example:

from enum import Enum

class A(Enum):
   x = 1
   x = 2

reveal_type(A.x)

Mypy is happy with it:

» mypy ex.py
ex.py:8: note: Revealed type is "Literal[ex.A.x]?"

But, it raises in runtime:

» python ex.py
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/mypy/ex.py", line 4, in <module>
    class A(Enum):
  File "/Users/sobolev/Desktop/mypy/ex.py", line 6, in A
    x = 2
  File "/Users/sobolev/.pyenv/versions/3.9.1/lib/python3.9/enum.py", line 99, in __setitem__
    raise TypeError('Attempted to reuse key: %r' % key)
TypeError: Attempted to reuse key: 'x'

This is a part of my "better enum" series:

• Marks enums with values as implicitly final #11247
• Makes Enum members implicitly final, refs #5599 #10852
• Enum plugin should support unique check where possible #10858

[sobolevn]

I will add a PR after #10858