This patch is fixing few errors related to Python3 and tests:
* Add the attribute "message" to the exception because it doesn't exist
by default.
* Add the py35 runtime to tox.ini since we now run tests on Python3.5
* Drop the spec=file syntax because "file" is not a builtin type for
Python3.
Change-Id: I31d1a5719d605974912bc26f65b96734631691d5
Closes-Bug: #1647337