Unblock migrate (py26 and py3* testing issues)

There are two changes which have to go together to pass the gate
tests:

1. Update pbr and mock requirements from global-requirements

mock 1.2 supports py26 again so make that the minimum version. The
same change is being made in g-r with:

Ic6b9e18eaec9c81bbbbc57129e024904be928e09

Sync up with latest pbr in global-requirements while we're at it.

Closes-Bug: #1474925

2. Fix the importpath module to work with python >= 3.3 where the
__import__ built-in is raising an ImportError on a temporary file
that is added to the system path.

Closes-Bug: #1475339

Change-Id: Ie98938ba75f3983094dd540b7d26a7ec46be4f6e

migrate/versioning/util/importpath.py

@@ -1,18 +1,30 @@
 import os
 import sys
 
-from six.moves import reload_module as reload
+PY33 = sys.version_info >= (3, 3)
+
+if PY33:
+    from importlib import machinery
+else:
+    from six.moves import reload_module as reload
+
 
 def import_path(fullpath):
     """ Import a file with full path specification. Allows one to
         import from anywhere, something __import__ does not do.
     """
-    # http://zephyrfalcon.org/weblog/arch_d7_2002_08_31.html
-    path, filename = os.path.split(fullpath)
-    filename, ext = os.path.splitext(filename)
-    sys.path.append(path)
-    module = __import__(filename)
-    reload(module) # Might be out of date during tests
-    del sys.path[-1]
-    return module
-
+    if PY33:
+        name = os.path.splitext(os.path.basename(fullpath))[0]
+        return machinery.SourceFileLoader(
+            name, fullpath).load_module(name)
+    else:
+        # http://zephyrfalcon.org/weblog/arch_d7_2002_08_31.html
+        path, filename = os.path.split(fullpath)
+        filename, ext = os.path.splitext(filename)
+        sys.path.append(path)
+        try:
+            module = __import__(filename)
+            reload(module)  # Might be out of date during tests
+            return module
+        finally:
+            del sys.path[-1]

requirements.txt

@@ -1,7 +1,7 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-pbr>=0.11,<2.0
+pbr>=1.3,<2.0
 
 # never put a cap on this, *ever*, sqla versions are handled via
 # tox, and if SQLA is capped it will only make it so we aren't testing

setup.py

@@ -16,6 +16,14 @@
 
 import setuptools
 
+# In python < 2.7.4, a lazy loading of package `pbr` will break
+# setuptools if some other modules registered functions in `atexit`.
+# solution from: http://bugs.python.org/issue15881#msg170215
+try:
+    import multiprocessing  # noqa
+except ImportError:
+    pass
+
 setuptools.setup(
-    setup_requires=['pbr'],
+    setup_requires=['pbr>=1.3'],
     pbr=True)

test-requirements.txt

@@ -8,7 +8,7 @@ coverage>=3.6
 discover
 feedparser
 fixtures>=0.3.14
-mock>=1.0
+mock>=1.2
 mox>=0.5.3
 psycopg2
 python-subunit>=0.0.18