Make typed decorators work for instance methods

This feels incomplete when compared to FunctionMixin.py__get__,
however seems to work at least in the cut-down reported.

Fixes https://github.com/davidhalter/jedi/issues/1801.

jedi/inference/gradual/typing.py

@@ -294,6 +294,9 @@ class Callable(BaseTypingInstance):
             from jedi.inference.gradual.annotation import infer_return_for_callable
             return infer_return_for_callable(arguments, param_values, result_values)
 
+    def py__get__(self, instance, class_value):
+        return ValueSet([self])
+
 
 class Tuple(BaseTypingInstance):
     def _is_homogenous(self):