`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.
Suggested-by: Rusty Russell <@rustyrussell> Suggested-by: Conor Scott <@conscott> Signed-off-by: Christian Decker <decker.christian@gmail.com>
Signed-off-by: Christian Decker <decker.christian@gmail.com>