www/py-txrequests: Allow Python 3.x builds

txrequests ships a universal (py2,py3) wheel in PyPI, which means its
Python 2/3 compatible and that the maintainer used bdist_wheel --universal
when uploading the wheel to PyPI. buildbot (2.2.0), which is now Python 3.x
only, depends on it for tests.

txrequests tests (3/3 of them) pass under Python 3, and buildbots test
utilising txrequests don't appear to fail.

Enable Python 3.x builds accordingly.