Properly type the ToJsonString method of time well known types in google protobuf (#2656)

Author: pcorpet

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

@@ -15,7 +15,7 @@ class Any_:
     def Is(self, descriptor: Any): ...
 
 class Timestamp:
-    def ToJsonString(self): ...
+    def ToJsonString(self) -> str: ...
     seconds: Any = ...
     nanos: Any = ...
     def FromJsonString(self, value: Any) -> None: ...
@@ -32,7 +32,7 @@ class Timestamp:
     def FromDatetime(self, dt: datetime) -> None: ...
 
 class Duration:
-    def ToJsonString(self): ...
+    def ToJsonString(self) -> str: ...
     seconds: Any = ...
     nanos: Any = ...
     def FromJsonString(self, value: Any) -> None: ...
@@ -48,7 +48,7 @@ class Duration:
     def FromTimedelta(self, td: Any) -> None: ...
 
 class FieldMask:
-    def ToJsonString(self): ...
+    def ToJsonString(self) -> str: ...
     def FromJsonString(self, value: Any) -> None: ...
     def IsValidForDescriptor(self, message_descriptor: Any): ...
     def AllFieldsFromDescriptor(self, message_descriptor: Any) -> None: ...