Use upstream provided source tarball, as building from git sources is
not required at all.
Switch to correct SPDX license identifier for upstream project.

Signed-off-by: David Runge <dvzrv@archlinux.org>

pypi tarball now ships the tests; wire them up

no longer used by calibre, and is therefore not needed at all

Fork of python-cssutils maintained by the calibre and sigil devs,
who are switching to require that instead.