import: pypi: Rewrite to use 'define-json-mapping'.

* guix/import/pypi.scm (non-empty-string-or-false): New procedure.
(<pypi-project>, <project-info>, <distribution>): New record types.
(pypi-fetch): Call 'json->pypi-project'.
(latest-source-release, latest-wheel-release): Use the new record
accessors instead of 'assoc-ref*'.
(pypi->guix-package, latest-release): Likewise.
* tests/pypi.scm (test-json): Add mandatory fields.

tests/pypi.scm

@@ -38,7 +38,10 @@
     \"license\": \"GNU LGPL\",
     \"summary\": \"summary\",
     \"home_page\": \"http://example.com\",
+    \"classifiers\": [],
+    \"download_url\": \"\"
   },
+  \"urls\": [],
   \"releases\": {
     \"1.0.0\": [
       {