package: read a specific Python version requirement file

This change makes pbr reads a specific requirement file tight to the
major Python version that is used. So you can add a file such as
requirements-py3.txt to have a specific requirement file for Python 3,
or requirements-py2.txt to have a specific requirement file for
Python 2.

Change-Id: I4b548e9830586f11b82539d334dd5cf4b0445a36