[ty] Recognize inherited enum member constructors

[charliermarsh]

Summary

When creating an enum class, CPython checks an inherited __new_member__ before falling back to an inherited __new__ or the data-type constructor. We previously skipped that lookup, so a parent enum could replace _value_ while we continued to expose the member's declared right-hand side as its precise .value type.

This change includes inherited __new_member__ in enum constructor discovery. When that hook is user-defined, member values now use the same conservative inference as other custom construction paths:

from enum import Enum

class Base(Enum):
    def __new_member__(cls: type["Base"], value: int) -> "Base":
        obj = object.__new__(cls)
        obj._value_ = str(value)
        return obj

class Child(Base):
    VALUE = 1

reveal_type(Child.VALUE.value)  # Any

This is split out from #26349 so constructor discovery can land independently of built-in enum mixin normalization.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.47%. The percentage of expected errors that received a diagnostic held steady at 89.19%. The number of fully passing files held steady at 95/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)

[carljm]

This doesn't match the runtime logic, in that the enum metaclass always saves __new__ as __new_member__, so it should be "walk the MRO and look for __new_member__ and then __new__ on each class encountered", not "first walk the MRO looking for __new_member__, then walk it looking for __new__".

Test case (I verified that at runtime it is always Parent.__new__ that is called, never Grandparent.__new_member__):

`EnumType` saves an enum's user-defined `__new__` as that class's `__new_member__`. The
immediate parent's `__new__` therefore takes precedence over an explicit `__new_member__`
in a grandparent:

```py
from enum import Enum

class Grandparent(Enum):
    def __new_member__(
        cls: type["Grandparent"], value: str
    ) -> "Grandparent":
        obj = object.__new__(cls)
        obj._value_ = value
        return obj

class Parent(Grandparent):
    def __new__(cls, value: int) -> "Parent":
        obj = object.__new__(cls)
        obj._value_ = value
        return obj

class Child(Parent):
    VALID = 1
    INVALID = "not an int"  # error: [invalid-assignment]

reveal_type(Child.VALID.value)  # revealed: Any

Pyright does get this right; pyrefly and mypy do not.

[charliermarsh]

Thanks!