LightningRpc should accept pathlib Path

Not sure why socket does not accept pathlib, but just added some code to transform the path to string.
It is not the most pretty code, but better than these:
https://stackoverflow.com/questions/58647584/how-to-test-if-object-is-a-pathlib-path

contrib/pyln-client/pyln/client/lightning.py

@@ -223,7 +223,7 @@ class UnixSocket(object):
     """
 
     def __init__(self, path: str):
-        self.path = path
+        self.path = str(path) if 'pathlib' in str(type(path)) else path
         self.sock: Optional[socket.SocketType] = None
         self.connect()