Strengthen typing of datetime methods of Google protobuf's Timestamp. (#2590)

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

@@ -1,4 +1,5 @@
 from typing import Any, Optional
+from datetime import datetime
 
 class Error(Exception): ...
 class ParseError(Error): ...
@@ -27,8 +28,8 @@ class Timestamp:
     def FromMicroseconds(self, micros: Any) -> None: ...
     def FromMilliseconds(self, millis: Any) -> None: ...
     def FromSeconds(self, seconds: Any) -> None: ...
-    def ToDatetime(self): ...
-    def FromDatetime(self, dt: Any) -> None: ...
+    def ToDatetime(self) -> datetime: ...
+    def FromDatetime(self, dt: datetime) -> None: ...
 
 class Duration:
     def ToJsonString(self): ...