Disable problematic requirements only on Python 3

[cclauss]

This issue is directly related to #1679, #1680, #1638, #1694, #1687 and indirectly related to #1522 and #1634.

Certain Python dependencies do not currently have an acceptable Python 3 equivalent and they block our Travis CI build process in the install step. This PR proposes to use the conditional capabilities of requirements.txt files to only install these modules on legacy Python and not install them on Python. This should allow us to get past the install step and start running our PyTests.

$ pip install -r requirements_test.txt

Ignoring Genshi: markers 'python_version < "3.0"' don't match your environment
Ignoring iptools: markers 'python_version < "3.0"' don't match your environment
Ignoring supervisor: markers 'python_version < "3.0"' don't match your environment
Ignoring OL-GeoIP: markers 'python_version < "3.0"' don't match your environment
Ignoring mockcache: markers 'python_version < "3.0"' don't match your environment

[mekarpeles]

I think this makes sense for now. We'll likely have to find alternatives for these packages in python3 eventually