>>> from contextlib import AbstractContextManager
>>> from typing import Protocol
>>> class Foo(AbstractContextManager, Protocol):
...     def extra_method(self) -> None: ...
...
>>>

>>> class Bar(Buffer, Protocol):
...     def extra_method(self) -> None: ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 106, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1916, in __init_subclass__
    raise TypeError('Protocols can only inherit from other'
TypeError: Protocols can only inherit from other protocols, got <class 'collections.abc.Buffer'>