pyln: Migrate remaining uses of the deprecated pylightning module

`pylightning` is not much more than an alias for `pyln-client`, so this
removes the need to install that as well just to run the tests.

tests/plugins/slow_init.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-from lightning import Plugin
+from pyln.client import Plugin
 import time
 
 plugin = Plugin()