pyln: Check to see if we have annotations on funcs before accessing

We were indiscriminately accessing the `__annotations__` which could cause
issues if the function had been wrapped by some functions such as
`functools.partial`. This just checks that the access is safe before doing it.

Suggested-by: jarret <@jarret>
Signed-off-by: Christian Decker <@cdecker>

contrib/pyln-client/pyln/client/plugin.py

@@ -317,9 +317,10 @@ class Plugin(object):
     @staticmethod
     def _coerce_arguments(func, ba):
         args = OrderedDict()
+        annotations = func.__annotations__ if hasattr(func, "__annotations__") else {}
         for key, val in ba.arguments.items():
-            annotation = func.__annotations__.get(key)
-            if annotation == Millisatoshi:
+            annotation = annotations.get(key, None)
+            if annotation is not None and annotation == Millisatoshi:
                 args[key] = Millisatoshi(val)
             else:
                 args[key] = val