Make HasField and ClearField use Text instead of str (#1425)

This allows one to write `x.HasField("ok")` even if the file has
`from __future__ import unicode_literals`"

Author: gvanrossum

third_party/2/google/protobuf/message.pyi

@@ -1,4 +1,4 @@
-from typing import Any, Sequence, Optional, Tuple
+from typing import Any, Sequence, Optional, Text, Tuple
 
 from .descriptor import FieldDescriptor
 
@@ -21,8 +21,8 @@ class Message:
     def SerializeToString(self) -> str: ...
     def SerializePartialToString(self) -> str: ...
     def ListFields(self) -> Sequence[Tuple[FieldDescriptor, Any]]: ...
-    def HasField(self, field_name: str) -> bool: ...
-    def ClearField(self, field_name: str) -> None: ...
+    def HasField(self, field_name: Text) -> bool: ...
+    def ClearField(self, field_name: Text) -> None: ...
     def WhichOneof(self, oneof_group) -> Optional[str]: ...
     def HasExtension(self, extension_handle): ...
     def ClearExtension(self, extension_handle): ...