Update type var typing on protobuf message.pyi (#3996)

Add support for message type var generics

third_party/2and3/google/protobuf/message.pyi

@@ -2,7 +2,7 @@ import sys
 
 from typing import Any, Sequence, Optional, Tuple, Type, TypeVar, Union
 
-from .descriptor import DescriptorBase, FieldDescriptor
+from .descriptor import Descriptor, DescriptorBase, FieldDescriptor
 
 class Error(Exception): ...
 class DecodeError(Error): ...
@@ -12,7 +12,7 @@ class _ExtensionDict:
     def __getitem__(self, extension_handle: DescriptorBase) -> Any: ...
     def __setitem__(self, extension_handle: DescriptorBase, value: Any) -> None: ...
 
-_T = TypeVar("_T")
+_M = TypeVar("_M", bound=Message)  # message type (of self)
 
 if sys.version_info < (3,):
     _Serialized = Union[bytes, buffer, unicode]
@@ -20,12 +20,12 @@ else:
     _Serialized = bytes
 
 class Message:
-    DESCRIPTOR: Any
+    DESCRIPTOR: Descriptor
     def __deepcopy__(self, memo=...): ...
     def __eq__(self, other_msg): ...
     def __ne__(self, other_msg): ...
-    def MergeFrom(self, other_msg: Message) -> None: ...
-    def CopyFrom(self, other_msg: Message) -> None: ...
+    def MergeFrom(self: _M, other_msg: _M) -> None: ...
+    def CopyFrom(self: _M, other_msg: _M) -> None: ...
     def Clear(self) -> None: ...
     def SetInParent(self) -> None: ...
     def IsInitialized(self) -> bool: ...
@@ -38,9 +38,10 @@ class Message:
     def ClearExtension(self, extension_handle): ...
     def ByteSize(self) -> int: ...
     @classmethod
-    def FromString(cls: Type[_T], s: _Serialized) -> _T: ...
+    def FromString(cls: Type[_M], s: _Serialized) -> _M: ...
     @property
     def Extensions(self) -> _ExtensionDict: ...
+
     # Intentionally left out typing on these three methods, because they are
     # stringly typed and it is not useful to call them on a Message directly.
     # We prefer more specific typing on individual subclasses of Message