Fix collections.abc deprecation warning

The ABCs in collections moved to the collections.abc module in 3.3 and
are not available from collections from 3.8 onwards. This fix should
work with any Python version (note 2.7 must also be supported).

Author: pgjones

h2/settings.py

@@ -15,6 +15,12 @@
 from h2.errors import ErrorCodes
 from h2.exceptions import InvalidSettingsValueError
 
+try:
+    from collections.abc import MutableMapping
+except ImportError:  # pragma: no cover
+    # Python 2.7 compatibility
+    from collections import MutableMapping
+
 
 class SettingCodes(enum.IntEnum):
     """
@@ -88,7 +94,7 @@ def __repr__(self):
         )
 
 
-class Settings(collections.MutableMapping):
+class Settings(MutableMapping):
     """
     An object that encapsulates HTTP/2 settings state.