Remove some ugly python2 compatibilty code from stix2.versioning

module, since we no longer support python2.

stix2/versioning.py

@@ -5,8 +5,7 @@ import datetime as dt
 import itertools
 import uuid
 
-import six
-from six.moves.collections_abc import Mapping
+from collections.abc import Mapping
 
 import stix2.base
 import stix2.registry
@@ -92,18 +91,9 @@ def _is_versionable(data):
 
         # Then, determine versionability.
 
-        if six.PY2:
-            # dumb python2 compatibility: map.keys() returns a list, not a set!
-            # six.viewkeys() compatibility function uses dict.viewkeys() on
-            # python2, which is not a Mapping mixin method, so that doesn't
-            # work either (for our stix2 objects).
-            keys = set(data)
-        else:
-            keys = data.keys()
-
         # This should be sufficient for STIX objects; maybe we get lucky with
         # dicts here but probably not.
-        if keys >= _VERSIONING_PROPERTIES:
+        if data.keys() >= _VERSIONING_PROPERTIES:
             is_versionable = True
 
         # Tougher to handle dicts.  We need to consider STIX version, map to a