gh-94808: Cover PyObject_PyBytes case with custom __bytes__ method

[sobolevn]

There were two options:

1. Add a new _testcapimodule.c function to test PyObject_PyBytes directly. But, there's already pyobject_bytes_from_null which only covers NULL case. So, it is somewhat covered
2. Add a test for some call that uses PyObject_Bytes, like int_from_bytes_impl. In this case we not only test C-API itself, but also do something useful for the end implementation (in this case int.from_bytes)

So, I went with 2.
Probably it is still possible to add a new _testcapimodule.c function in the next PR.

• Issue: Metabug: Improving C-level coverage #94808