python3Packages.pytaglib: fix build

Applied several patches to fix the test suite on python 2.7 and to
properly install the `pyprinttags` executable. Also switched to the
GitHub source for now as the PyPI tarball was wrongly packaged and
didn't contain the `pyprinttags.py` script (see the last two patches for
further reference).

See also https://hydra.nixos.org/build/102493330

ZHF #68361