Make sure py__get__ is defined on all values

Also define matches_signature on all signatures, there's definitely cases where that might be called

jedi/inference/signature.py

@@ -56,6 +56,9 @@ class AbstractSignature(_SignatureMixin):
     def bind(self, value):
         raise NotImplementedError
 
+    def matches_signature(self, arguments):
+        return True
+
     def __repr__(self):
         if self.value is self._function_value:
             return '<%s: %s>' % (self.__class__.__name__, self.value)