pylightning: support functions with type annotations.

I tried annotating the plugin-millisatoshis.py plugin, and it failed like so:

	plugin-millisatoshis.py Killing plugin: "getmanifest" result is not an object: {"jsonrpc": "2.0", "id": 1, "error": "Error while processing getmanifest: ValueError(\'Function has keyword-only parameters or annotations, use getfullargspec() API which can support them\',)"}'

So, let's do that!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pylightning/lightning/plugin.py

@@ -451,7 +451,7 @@ class Plugin(object):
 
             # Handles out-of-order use of parameters like:
             # def hello_obfus(arg1, arg2, plugin, thing3, request=None, thing5='at', thing6=21)
-            argspec = inspect.getargspec(method.func)
+            argspec = inspect.getfullargspec(method.func)
             defaults = argspec.defaults
             num_defaults = len(defaults) if defaults else 0
             start_kwargs_idx = len(argspec.args) - num_defaults