bumps dependencies

We need newer model and web client to operate.

requirements-swh.txt

 # Add here internal Software Heritage dependencies, one per line.
 swh.core >= 0.3
-swh.model >= 6.13.0
+swh.model >= 6.15.0
 swh.auth
-swh.web.client
+swh.web.client >= 0.9.0