Allow multiple requirements files to be specified.

Some projects have more than one requirements file: this is the case for
OpenStack projects, which usually have requirements.txt, test-requirements.txt
and sometimes Python3 specific requirements.

This patch allows users to specify multiple requirements file by using this
syntax:

    caniusepython3 -r req1.txt,req2.txt,req3.txt

The behaviour is unchanged if a single file is used.