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
8252703f56