Do not fallback when importing "unittest.mock"

[nicoddemus]

try:
    import mock as mock_module
except ImportError:
    import unittest.mock as mock_module

Use this instead:

if sys.version_info.major == 2:
    import mock as mock_module
else:
    import unittest.mock as mock_module

This way we don't "swallow" the first exception in Python 2 in case something goes wrong when importing mock.

EDIT: Just for the record, the first import mock as mock_module was raising an ImportError but not because mock was missing, one of its dependencies was not installed (it was a frozen application). This made this problem much harder to debug.

cc @gqmelo