Use a separate type variable for unique()

[gvanrossum]

See #1136 (comment)

[gvanrossum]

@JelleZijlstra This what you meant?

[JelleZijlstra]

Yes, thanks! I am going to try this out to see if it works properly in all cases (but don't have time right now).

[JelleZijlstra]

Seems to work:

$ cat ../bin/enumu.py 
import enum

class X(enum.Enum):
    a = 1
    b = 2
reveal_type(enum.unique(X))

@enum.unique
class Y(enum.Enum):
    c = 3
    d = 4
reveal_type(Y)
$ mypy --custom-typeshed-dir . ../bin/enumu.py 
../bin/enumu.py:6: error: Revealed type is 'def (value: Any) -> enumu.X'
../bin/enumu.py:12: error: Revealed type is 'def (value: Any) -> enumu.Y'