Use a symmetric type and make mypy happy
@@ -358,7 +358,7 @@ class RequirementPreparer(object):
self._downloaded = {} # type: Dict[str, Tuple[str, str]]
# Previous "header" printed for a link-based InstallRequirement
- self._previous_requirement_header = None
+ self._previous_requirement_header = ("", "")
@property
def _download_should_save(self):
