Some of the builtin methods are not recognized as such by inspect.isroutine(). inspect.ismethoddescriptor() only returns True for the unbound versions of the methods but not the bound ones.

For example:

>>> inspect.isroutine(object.__str__)
True
>>> inspect.isroutine(object().__str__)
False

The second one should also return True as it does for user-defined classes:

>>> class A:
...     def f(self):
...         pass
>>> inspect.isroutine(A.f)
True
>>> inspect.isroutine(A().f)
True

The types needed for that have already been added to the types module in issue #29377. Here is the commit: https://github.com/python/cpython/commit/1947d33a0a1c2ba006397579ec6235528faea9fd