add outputs python-version and python-cache-hit (#728)

This commit splits up the "normal" cache containing the dependencies and
the "python" cache containing the python binaries. This will lead to a
one-time invalidation of caches.

Closes: #713

action-types.yml

@@ -69,3 +69,7 @@ outputs:
     type: string
   venv:
     type: string
+  python-version:
+    type: string
+  python-cache-hit:
+    type: boolean