fix type-check error (Optional requires type argument) in python 3.6+
@@ -152,7 +152,7 @@ class KeyValueStorageFile(KeyValueStorage):
@abstractmethod
def _parse_line(self, line, prefix=None, returnKey: bool=True,
- returnValue: bool=True, key=None) -> Tuple[Optional, Optional]:
+ returnValue: bool=True, key=None) -> Tuple[Optional[str], Optional[str]]:
pass
@abstractmethod
