Here's a minimal reproduction:

» ./python.exe
Python 3.12.0a1+ (heads/main:bded5edd9a, Oct 27 2022, 23:29:21) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class SubStr(str): ...
... 
>>> int.from_bytes(b"0", SubStr("big"))
Assertion failed: (PyUnicode_CheckExact(str1)), function _PyUnicode_Equal, file Objects/unicodeobject.c, line 10447.
[1]    18466 abort      ./python.exe

I think this happes because of these lines:

_PyUnicode_Equal uses assert(PyUnicode_CheckExact(...)), while many function (including int_from_bytes_impl) use PyUnicode_Check() or just parse str objects from args.

Probably other functions that use _PyUnicode_Equal are also affected.

I would like to raise a question: shouldn't it be assert(PyUnicode_Check(...)) in _PyUnicode_Equal?

I would like to send a PR with the fix!