import: pypi: Gracefully handle wrong argument counts.

* guix/scripts/import/pypi.scm (guix-import-pypi): Use 'leave' to handle
  cases where ARGS has zero or two or more elements.

guix/scripts/import/pypi.scm

@@ -84,4 +84,8 @@ Import and convert the PyPI package for PACKAGE-NAME.\n"))
          (unless sexp
            (leave (_ "failed to download meta-data for package '~a'~%")
                   package-name))
-         sexp)))))
+         sexp))
+      (()
+       (leave (_ "too few arguments~%")))
+      ((many ...)
+       (leave (_ "too many arguments~%"))))))