Make EnumTypeWrapper generic in google.protobuf.internal (#3995)

This is necessary so that mypy-protobuf can autogenerate NewType
wrappers around the int values of the enum!

third_party/2and3/google/protobuf/internal/enum_type_wrapper.pyi

@@ -1,10 +1,19 @@
-from typing import Any, List, Tuple
+from typing import Any, Generic, List, Tuple, TypeVar
 
-class EnumTypeWrapper(object):
-    def __init__(self, enum_type: Any) -> None: ...
-    def Name(self, number: int) -> bytes: ...
-    def Value(self, name: bytes) -> int: ...
-    def keys(self) -> List[bytes]: ...
-    def values(self) -> List[int]: ...
-    @classmethod
-    def items(cls) -> List[Tuple[bytes, int]]: ...
+from google.protobuf.descriptor import EnumDescriptor
+
+_V = TypeVar("_V", bound=int)
+
+# Expose a generic version so that those using mypy-protobuf
+# can get autogenerated NewType wrapper around the int values
+class _EnumTypeWrapper(Generic[_V]):
+    DESCRIPTOR: EnumDescriptor
+
+    def __init__(self, enum_type: EnumDescriptor) -> None: ...
+    def Name(self, number: _V) -> str: ...
+    def Value(self, name: str) -> _V: ...
+    def keys(self) -> List[str]: ...
+    def values(self) -> List[_V]: ...
+    def items(self) -> List[Tuple[str, _V]]: ...
+
+class EnumTypeWrapper(_EnumTypeWrapper[int]): ...