Fixes <https://bugs.gnu.org/25635>.
Reported by Sergei Trofimovich <slyfox@inbox.ru>.
* guix/import/pypi.scm (guix-package->pypi-name)[url->pypi-name]: New
procedure.
Rewrite body to match lists in addition to strings.
* tests/pypi.scm ("guix-package->pypi-name, several URLs"): New test.