python/qrcode: Add mock to checkInputs

Another test issue surfacing from the setuptools bump in
70c18997

.

This time, the tests just fail because mock is missing as a dependency.

Tested building against Python 2.7 and Python 3.6.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @joachifm, @Mic92