Remove version check that is not much needed anymore (for DefaultDict)

Author: rhettinger

pubsub/pubsub.py

@@ -1,8 +1,5 @@
 'Simple message publisher/subscriber service'
 
-import sys
-assert sys.version_info[:3] >= (3, 6, 1), "Requires 3.6.1 or later"
-
 from typing import List, Tuple, DefaultDict, Set, Optional, NamedTuple, Deque, Dict
 from collections import deque, defaultdict
 import hashlib