gh-120593: Fix const qualifier in _PyLong_CompactValue()

[vstinner]

Remove the const qualifier of the argument of functions:

• _PyLong_IsCompact()
• _PyLong_CompactValue()

Py_TYPE() argument is not const.

Fix the compiler warning:

  Include/cpython/longintrepr.h: In function ‘_PyLong_CompactValue’: Include/pyport.h:19:31: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  (...) 
  Include/cpython/longintrepr.h:133:30: note: in expansion of macro ‘Py_TYPE’ assert(PyType_HasFeature(Py_TYPE(op), Py_TPFLAGS_LONG_SUBCLASS));

• Issue: pyatomic_gcc.h with GCC discards const qualifier  #120593

[vstinner]

The important part is to respect the -Werror=cast-qual flag (avoid compiler warnings) in the public Python C API.

In Python implementation and the internal C API, we don't have to worry about this flag, since we don't use it.

[miss-islington-app]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @vstinner, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker e51e880e75d79687b54a71351266e29ee349b4b8 3.13

[vstinner]

The backport to Python 3.13 is not needed.

[markshannon]

The const modifier is correct.
Please don't remove them, they add some much needed type safety to our C code.
If Py_TYPE() argument is not const, then fix that, don't weaken the types of other code.

[vstinner]

The const modifier is correct. Please don't remove them, they add some much needed type safety to our C code.

Ok, I restored the const modified with PR gh-122367.

If Py_TYPE() argument is not const, then fix that, don't weaken the types of other code.

I tried hard to use const PyObject*, but it went badly: #91768 I'm not convinced that it helps the compiler to optimize.

In my PR, I just restored the code to avoid Py_TYPE().