Fixes <http://bugs.gnu.org/23062>.
Reported by Danny Milosavljevic <dannym@scratchpost.org>.
* guix/import/pypi.scm (make-pypi-sexp): Check whether 'pypi-uri'
returns SOURCE-URL and fall back to the full URL otherwise.
* tests/pypi.scm ("pypi->guix-package"): Adjust expected URI
accordingly.
Co-authored-by: Danny Milosavljevic <dannym@scratchpost.org>