stubgenc.py throw TypeError: unbound method type.mro() needs an argument

[gen-xu]

Bug Report

When I was trying to use stubgen to generate pyi for the PyAV package, an error is being thrown.

  File "mypy\stubgenc.py", line 315, in generate_c_type_stub
TypeError: unbound method type.mro() needs an argument

After a bit investigation in stubgenc.py and PyAV. I found the issue is caused by these
https://github.com/python/mypy/blob/master/mypy/stubgenc.py#L299
https://github.com/python/mypy/blob/master/mypy/stubgenc.py#L355
Basically, when invoking obj.mro() with EnumType defined in https://github.com/PyAV-Org/PyAV/blob/main/av/enum.pyx#L25, it resolves into <type>.mro() and complains missing argument, because EnumType inherits type.
The minimum case of this would just be python -c 'type.mro()'

To Reproduce

1. Install PyAV with pip install av I am using 8.0.3 but most versions should work though
2. Install mypy with pip install mypy I am using 0.812 but most versions with stubgen should behave similar
3. Run command stubgen -p av or more specifically stubgen -m av.enum

Expected Behavior

It supposed to generate pyi file without throwing errors

Actual Behavior

PS C:\Users\Gen\Repos> stubgen -m av.enum
Traceback (most recent call last):
  File "c:\users\gen\appdata\local\programs\python\python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\users\gen\appdata\local\programs\python\python39\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\Gen\AppData\Local\Programs\Python\Python39\Scripts\stubgen.exe\__main__.py", line 7, in <module>
  File "mypy\stubgen.py", line 1582, in main
  File "mypy\stubgen.py", line 1475, in generate_stubs
  File "mypy\stubgenc.py", line 64, in generate_stub_for_c_module
  File "mypy\stubgenc.py", line 315, in generate_c_type_stub
TypeError: unbound method type.mro() needs an argument

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: stubgen -m av.enum
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9.1
• Operating system and version: Windows 10 Pro 19042.804