Fix Enum type definition (#8624)

Using the : syntax leads to non member attributes.

> If an attribute is defined in the class body with a type annotation
> but with no assigned value, a type checker should assume this is a non-member attribute

```
class Pet(Enum):
    genus: str  # Non-member attribute
    species: str  # Non-member attribute

    CAT = 1  # Member attribute
    DOG = 2  # Member attribute
```

https://typing.python.org/en/latest/spec/enums.html#defining-members

tests/MyGame/Example/NestedUnion/Any.pyi

@@ -8,12 +8,13 @@ import typing
 from MyGame.Example.NestedUnion.TestSimpleTableWithEnum import TestSimpleTableWithEnum
 from MyGame.Example.NestedUnion.Vec3 import Vec3
 from flatbuffers import table
+from typing import cast
 
 uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
 
 class Any(object):
-  NONE: int
-  Vec3: int
-  TestSimpleTableWithEnum: int
+  NONE = cast(int, ...)
+  Vec3 = cast(int, ...)
+  TestSimpleTableWithEnum = cast(int, ...)
 def AnyCreator(union_type: typing.Literal[Any.NONE, Any.Vec3, Any.TestSimpleTableWithEnum], table: table.Table) -> typing.Union[None, Vec3, TestSimpleTableWithEnum]: ...