Improve testenv for py26
    
    Original testenv for py26 works well, but it requires newer version of
    setuptools which is not avaialbe in EL6 and EL7 and that causes failure
    to build EPEL 6 and 7 packages. Instead of making changes to SPEC file,
    this patch improves the test env provision for py26 specifically in
    tox.ini, the result would make it work in both tox and building the RPM
    packages.
    
    * pypi.txt and test-pypi.txt does not include any platform specifier
      and only list packages that rpkg requires.
    * install_requires and tests_require are modified in setup.py according
      to the Python version. Specific packages that are required for Python
      2.6 are added.
    * Provision py26 testenv specifically in tox.ini.
    * Tests still run with `GitPython<1.0.0` in Python 2.6.
    * Comments are cleaned in pypi.txt.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>