Fixes <http://bugs.gnu.org/23997>.
* guix/import/pypi.scm (guix-package->pypi-name): Rewrite using
'basename' and 'hyphen-package-name->name+version'.
* tests/pypi.scm ("guix-package->pypi-name, old URL style")
("guix-package->pypi-name, new URL style"): New tests.