Handle collections.abc deprecations

The use of ABC classes directly from collections has been deprecated in
3.x versions of Python. The direction is to use the classes defined in
collections.abc. Python 2.7 does not have this, but Python 3.8 will be
dropping the backwards compatibility to use the old location.

Six also does not have support for this yet, so in the mean time to make
sure we don't run into issues as folks try to move to 3.8, and to get
rid of deprecation warnings in logs, this handles importing from the
preferred location and falls back if it not available.

Change-Id: I535640779da645e43ba6586308b07bf25dba7d89
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

oslo_config/cfg.py

@@ -17,6 +17,14 @@
 
 import argparse
 import collections
+
+# TODO(smcginnis) update this once six has support for collections.abc
+# (https://github.com/benjaminp/six/pull/241) or clean up once we drop py2.7.
+try:
+    from collections.abc import Mapping
+except ImportError:
+    from collections import Mapping
+
 import copy
 import errno
 import functools
@@ -1910,7 +1918,7 @@ class _CachedArgumentParser(argparse.ArgumentParser):
         super(_CachedArgumentParser, self).print_usage(file)
 
 
-class ConfigOpts(collections.Mapping):
+class ConfigOpts(Mapping):
 
     """Config options which may be set on the command line or in config files.
 
@@ -3099,7 +3107,7 @@ class ConfigOpts(collections.Mapping):
         value, loc = self._do_get(name, opt_group, None)
         return loc
 
-    class GroupAttr(collections.Mapping):
+    class GroupAttr(Mapping):
 
         """Helper class.