Remove 3.6-related code.

Author: Julian

jsonschema/_format.py

@@ -354,13 +354,6 @@ def is_regex(instance):
     return re.compile(instance)
 
 
-if hasattr(datetime.date, "fromisoformat"):
-    _is_date = datetime.date.fromisoformat
-else:
-    def _is_date(instance):
-        return datetime.datetime.strptime(instance, "%Y-%m-%d")
-
-
 @_checks_drafts(
     draft3="date",
     draft7="date",