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/TestEnum.pyi

@@ -5,11 +5,12 @@ import numpy as np
 
 import flatbuffers
 import typing
+from typing import cast
 
 uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
 
 class TestEnum(object):
-  A: int
-  B: int
-  C: int
+  A = cast(int, ...)
+  B = cast(int, ...)
+  C = cast(int, ...)