`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.
We don't, but we should, like we do for normal RPC. However, I chose to use function annotations, rather than names-ending-in-'msat' because it's more Pythony. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>