tzinfo.tzname is optional (#1934)

Author: kennydo

stdlib/2and3/datetime.pyi

@@ -14,7 +14,7 @@ MINYEAR: int
 MAXYEAR: int
 
 class tzinfo:
-    def tzname(self, dt: Optional[datetime]) -> str: ...
+    def tzname(self, dt: Optional[datetime]) -> Optional[str]: ...
     def utcoffset(self, dt: Optional[datetime]) -> Optional[timedelta]: ...
     def dst(self, dt: Optional[datetime]) -> Optional[timedelta]: ...
     def fromutc(self, dt: datetime) -> datetime: ...