Use _PyObject_CAST in internal headers

[clin1234]

To be resolved by #135892

Linked PRs

• gh-135906: Use _PyObject_CAST in internal CPython headers #135892
• gh-135906: Test the internal C API in test_cext #136247
• [3.14] gh-135906: Use _PyObject_CAST in internal headers (GH-135892) #136392

[ZeroIntensity]

Keeping this open until we figure out how to test for this.