import: PyPI: Update redirected URL.
* guix/import/pypi.scm (guix-package->pypi-name, pypi->guix-package): Update
docstrings.
(pypi-package?): Test for pypi.org, too.
(pypi-fetch): s/pypi.python.org/pypi.org/
* tests/pypi.scm ("guix-package->pypi-name, new URL style",
"pypi->guix-package", "pypi->guix-package, wheels"): Likewise.