Use _typeshed.Self in protobuf/google/protobuf/message.pyi (#6955)

stubs/protobuf/google/protobuf/message.pyi

@@ -1,3 +1,4 @@
+from _typeshed import Self
 from typing import Any, Sequence, TypeVar
 
 from .descriptor import Descriptor, FieldDescriptor
@@ -14,8 +15,8 @@ class Message:
     def __deepcopy__(self, memo=...): ...
     def __eq__(self, other_msg): ...
     def __ne__(self, other_msg): ...
-    def MergeFrom(self: _M, other_msg: _M) -> None: ...
-    def CopyFrom(self: _M, other_msg: _M) -> None: ...
+    def MergeFrom(self: Self, other_msg: Self) -> None: ...
+    def CopyFrom(self: Self, other_msg: Self) -> None: ...
     def Clear(self) -> None: ...
     def SetInParent(self) -> None: ...
     def IsInitialized(self) -> bool: ...
@@ -24,11 +25,13 @@ class Message:
     def SerializeToString(self, deterministic: bool = ...) -> bytes: ...
     def SerializePartialToString(self, deterministic: bool = ...) -> bytes: ...
     def ListFields(self) -> Sequence[tuple[FieldDescriptor, Any]]: ...
+    # The TypeVar must be bound to `Message` or we get mypy errors, so we cannot use `Self` for `HasExtension` & `ClearExtension`
     def HasExtension(self: _M, extension_handle: _ExtensionFieldDescriptor[_M, Any]) -> bool: ...
     def ClearExtension(self: _M, extension_handle: _ExtensionFieldDescriptor[_M, Any]) -> None: ...
     def ByteSize(self) -> int: ...
     @classmethod
-    def FromString(cls: type[_M], s: bytes) -> _M: ...
+    def FromString(cls: type[Self], s: bytes) -> Self: ...
+    # The TypeVar must be bound to `Message` or we get mypy errors, so we cannot use `Self` for `Extensions`
     @property
     def Extensions(self: _M) -> _ExtensionDict[_M]: ...
     # Intentionally left out typing on these three methods, because they are