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.