Fix enum attributes are not members

[terencehonles]

This adds on to the change in #17182 and fixes enum attributes being used as members.

@hamdanal / @hauntsaninja I noticed there was the function get_enum_values in mypy types and instead of adding more to the case in try_expanding_sum_type_to_union it seemed like it might make sense to move the changes from #17182 there. I don't think the other code touched in that PR can use get_enum_values.

[hamdanal]

This last condition changes the behavior of mypy which currently considers unassigned but annotated attributes as members of the enum.
I would say the new behavior is better but it must be noted in the PR title or description that this is a breaking change that must be considered on its own.
Looking at the mypy primer output, it looks like members of enums constructed using the enum call syntax do not have the has_explicit_value set which breaks type narrowing. I suggest changing the variable here to also set this flag perhaps with a comment explaining the reason -- something like "E = Enum("E", "A") is equivalent to class E(Enum): A = auto()"

[terencehonles]

Thanks for noticing that, I've not really used them in that form, but hopefully the comment is not too wordy. I only glanced at the code that you pointed to, but I was cross referencing https://github.com/python/cpython/blob/7528b84e947f727734bd802356e380673553387d/Lib/enum.py#L828-L839 and how much of this is implemented by mypy? 🤔

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
+ tanjun/_internal/__init__.py:189: error: Statement is unreachable  [unreachable]
+ tanjun/annotations.py:2646: error: Statement is unreachable  [unreachable]

[terencehonles]

It looks like this is actually related to the ongoing discussion python/typing-council#11