remotenames: work around move of ABCs in collections

This starts warning in Python 3.7, and will break in 3.8. The import
is performed in a curious place because `import collections.abc`
explodes badly on Python 2.7, and `from collections import abc` flunks
our import checker.

Differential Revision: https://phab.mercurial-scm.org/D2847

hgext/remotenames.py

@@ -22,8 +22,6 b' remotenames.branches'
 
 from __future__ import absolute_import
 
-import collections
-
 from mercurial.i18n import _
 
 from mercurial.node import (
@@ -32,12 +30,20 b' from mercurial.node import ('
 from mercurial import (
     logexchange,
     namespaces,
+    pycompat,
     registrar,
     revsetlang,
     smartset,
     templateutil,
 )
 
+if pycompat.ispy3:
+    import collections.abc
+    mutablemapping = collections.abc.MutableMapping
+else:
+    import collections
+    mutablemapping = collections.MutableMapping
+
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
@@ -56,7 +62,7 b" configitem('remotenames', 'branches',"
     default=True,
 )
 
-class lazyremotenamedict(collections.MutableMapping):
+class lazyremotenamedict(mutablemapping):
     """
     Read-only dict-like Class to lazily resolve remotename entries