gh-120593: Fix const qualifier in _PyLong_CompactValue() (#121053)

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));