Update for recent Python version

At some point, apparently Python 3.3, collections.Mapping moved to
collections.abc.Mapping [0]. Update the import accordingly.

[0] https://docs.python.org/3/library/collections.abc.html

sphinxcontrib/openapi/renderers/_httpdomain.py

@@ -1,6 +1,11 @@
 """OpenAPI spec renderer."""
 
-import collections
+import sys
+if sys.version_info >= (3, 3):
+    import collections.abc as collections
+else:
+    import collections
+
 import copy
 import functools
 import http.client