[ty] Detect enums if metaclass is a subtype of EnumType/EnumMeta (#19481)

## Summary

This PR implements the following section from the [typing spec on
enums](https://typing.python.org/en/latest/spec/enums.html#enum-definition):

> Enum classes can also be defined using a subclass of `enum.Enum` **or
any class that uses `enum.EnumType` (or a subclass thereof) as a
metaclass**. Note that `enum.EnumType` was named `enum.EnumMeta` prior
to Python 3.11.

part of https://github.com/astral-sh/ty/issues/183

## Test Plan

New Markdown tests

crates/ty_python_semantic/resources/mdtest/enums.md

@@ -561,7 +561,83 @@ reveal_type(enum_members(Answer))
 
 ## Custom enum types
 
-To do: <https://typing.python.org/en/latest/spec/enums.html#enum-definition>
+Enum classes can also be defined using a subclass of `enum.Enum` or any class that uses
+`enum.EnumType` (or a subclass thereof) as a metaclass. `enum.EnumType` was called `enum.EnumMeta`
+prior to Python 3.11.
+
+### Subclasses of `Enum`
+
+```py
+from enum import Enum, EnumMeta
+
+class CustomEnumSubclass(Enum):
+    def custom_method(self) -> int:
+        return 0
+
+class EnumWithCustomEnumSubclass(CustomEnumSubclass):
+    NO = 0
+    YES = 1
+
+reveal_type(EnumWithCustomEnumSubclass.NO)  # revealed: Literal[EnumWithCustomEnumSubclass.NO]
+reveal_type(EnumWithCustomEnumSubclass.NO.custom_method())  # revealed: int
+```
+
+### Enums with (subclasses of) `EnumMeta` as metaclass
+
+```toml
+[environment]
+python-version = "3.9"
+```
+
+```py
+from enum import Enum, EnumMeta
+
+class EnumWithEnumMetaMetaclass(metaclass=EnumMeta):
+    NO = 0
+    YES = 1
+
+reveal_type(EnumWithEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithEnumMetaMetaclass.NO]
+
+class SubclassOfEnumMeta(EnumMeta): ...
+
+class EnumWithSubclassOfEnumMetaMetaclass(metaclass=SubclassOfEnumMeta):
+    NO = 0
+    YES = 1
+
+reveal_type(EnumWithSubclassOfEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithSubclassOfEnumMetaMetaclass.NO]
+
+# Attributes like `.value` can *not* be accessed on members of these enums:
+# error: [unresolved-attribute]
+EnumWithSubclassOfEnumMetaMetaclass.NO.value
+```
+
+### Enums with (subclasses of) `EnumType` as metaclass
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+```py
+from enum import Enum, EnumType
+
+class EnumWithEnumMetaMetaclass(metaclass=EnumType):
+    NO = 0
+    YES = 1
+
+reveal_type(EnumWithEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithEnumMetaMetaclass.NO]
+
+class SubclassOfEnumMeta(EnumType): ...
+
+class EnumWithSubclassOfEnumMetaMetaclass(metaclass=SubclassOfEnumMeta):
+    NO = 0
+    YES = 1
+
+reveal_type(EnumWithSubclassOfEnumMetaMetaclass.NO)  # revealed: Literal[EnumWithSubclassOfEnumMetaMetaclass.NO]
+
+# error: [unresolved-attribute]
+EnumWithSubclassOfEnumMetaMetaclass.NO.value
+```
 
 ## Function syntax