[Python] Add missing Python kwargs builtin support

Accept keyword arguments accessing C++ static member functions when
using -builtin and kwargs feature and Python class staticmethod syntax.
The missing keyword argument support was only when using the
class staticmethod syntax, not when using the flat static method
syntax.

Examples/test-suite/python/kwargs_feature_runme.py

@@ -18,6 +18,9 @@ if f.foo(b=1, a=2) != 3:
 if Foo.statfoo(b=2) != 3:
     raise RuntimeError
 
+if Foo.statfoo_onearg(x=4) != 8:
+    raise RuntimeError
+
 if f.efoo(b=2) != 3:
     raise RuntimeError