cli: Use temporary scheduler as fallback when no configuration detected

In order to simplify the testing of listers, allow to call the run command
of swh-lister CLI without scheduler configuration. In that case a temporary
scheduler instance with a postgresql backend is created and used.

It enables to easily test a lister with the following command:

$ swh -l DEBUG lister run <lister_name> url=<forge_url>

requirements-swh.txt

@@ -1,2 +1,2 @@
 swh.core[db,github] >= 2.22.0
-swh.scheduler >= 0.8
+swh.scheduler >= 1.12.0