pycompat: add empty and queue to handle py3 divergence

While the pycompat module will actually handle divergence, please
access these properties from the util module:
util.queue = Queue.Queue / queue.Queue
util.empty = Queue.Empty / queue.Empty

mercurial/pycompat.py

@@ -0,0 +1,18 b''
+# pycompat.py - portability shim for python 3
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""Mercurial portability shim for python 3.
+
+This contains aliases to hide python version-specific details from the core.
+"""
+
+from __future__ import absolute_import
+
+try:
+    import Queue as _queue
+except ImportError:
+    import queue as _queue
+empty = _queue.Empty
+queue = _queue.Queue

mercurial/util.py

@@ -43,8 +43,15 b' from . import ('
     i18n,
     osutil,
     parsers,
+    pycompat,
 )
 
+for attr in (
+    'empty',
+    'queue',
+):
+    globals()[attr] = getattr(pycompat, attr)
+
 if os.name == 'nt':
     from . import windows as platform
 else: