Fix "DeprecationWarning" from "collections" (#451)

DeprecationWarning: Using or importing the ABCs from 'collections'
instead of from 'collections.abc' is deprecated, and in 3.8 it will stop
working.

Author: tretinha

src/cloudant/_common_util.py

@@ -19,7 +19,7 @@
 
 import sys
 import platform
-from collections import Sequence
+from collections.abc import Sequence
 import json
 
 from ._2to3 import LONGTYPE, STRTYPE, NONETYPE, UNITYPE, iteritems_