Update to pyln-client

This updates all plugins *which already have requirements.txt* to
pyln-client from pylightning.

drain/drain.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-from lightning import Plugin, Millisatoshi, RpcError
+from pyln.client import Plugin, Millisatoshi, RpcError
 import re
 import time
 import uuid

drain/requirements.txt

@@ -1 +1 @@
-pylightning>=0.0.7.3
+pyln-client>=0.7.3