gh-120593: Fix const qualifier in pyatomic.h

[vstinner]

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

[vstinner]

This PR fix the following warnings:

/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h: In function ‘_Py_atomic_load_ptr’:
/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h:300:34: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  300 | { return (void *)__atomic_load_n((void **)obj, __ATOMIC_SEQ_CST); }
      |                                  ^

/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h: In function ‘_Py_atomic_load_ptr_relaxed’:
/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h:359:34: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  359 | { return (void *)__atomic_load_n((const void **)obj, __ATOMIC_RELAXED); }
      |                                  ^

/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h:359:10: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  359 | { return (void *)__atomic_load_n((const void **)obj, __ATOMIC_RELAXED); }
      |          ^
/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h: In function ‘_Py_atomic_load_ptr_acquire’:
/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h:493:34: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  493 | { return (void *)__atomic_load_n((void **)obj, __ATOMIC_ACQUIRE); }
      |                                  ^

/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h: In function ‘_Py_atomic_load_uintptr_acquire’:
/home/vstinner/python/main/Include/cpython/pyatomic_gcc.h:497:37: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
  497 | { return (uintptr_t)__atomic_load_n((uintptr_t *)obj, __ATOMIC_ACQUIRE); }
      |                                     ^

cc1: all warnings being treated as errors
error: command '/usr/bin/gcc' failed with exit code 1

[vstinner]

My previous attempt using (const void * const *) cast didn't work well. The correct cast is to (void * const *).

[colesbury]

LGTM!

[miss-islington-app]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-121057 is a backport of this pull request to the 3.13 branch.