Stubtest: check _value_ for ellipsis-valued stub enum members

[sterliakov]

Currently stubtest allows unsound definitions:

# a.pyi
from enum import Enum
class E(Enum):
    _value_: str
    FOO = ...

# a.py
from enum import Enum
class E(Enum):
    FOO = 0

This PR teaches stubtest that _value_ attribute (spec) should be used as a fallback in such case.

[A5rocks]

Is this intentionally making the runtime_type is None case error now? (I tried thinking about it but IDK what that means. I also don't know stubtest very well :^)

[sterliakov]

I also tried thinking about that, couldn't invent a case where we have found an attribute but failed to interpret its value, and decided to be cautious, reporting such "smth weird" cases as incompatibilities. Maybe this should be an assert, but I'm afraid to accidentally kill stubtest right before release...

[sterliakov]

Okay, almost all other checks do not reject "wtf" values as incompatible, updated for consistency.