Use PY_VERSION_HEX for version comparison

`PY_VERSION_HEX` should be used when comparing version numbers, for
example `PY_VERSION_HEX >= 0x03080000`.

Avoid `PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 8` because
that is true for 3.8-3.12, but also for 4.8-4.12 and so on.

Hoist the comment from `PY_VERSION_HEX`'s definition in `patchlevel.h`
to docs in `apiabiversion.rst`.

Author: hugovk

Doc/c-api/apiabiversion.rst

@@ -58,6 +58,8 @@ See :ref:`stable` for a discussion of API and ABI stability across versions.
    Thus ``3.4.1a2`` is hexversion ``0x030401a2`` and ``3.10.0`` is
    hexversion ``0x030a00f0``.
 
+   Use this for numeric comparisons, e.g. ``#if PY_VERSION_HEX >= ...``
+
    This version is also available via the symbol :data:`Py_Version`.
 
 .. c:var:: const unsigned long Py_Version

Doc/whatsnew/3.11.rst

@@ -2319,7 +2319,7 @@ Porting to Python 3.11
   can define the following macros and use them throughout
   the code (credit: these were copied from the ``mypy`` codebase)::
 
-    #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 8
+    #if PY_VERSION_HEX >= 0x03080000
     #  define CPy_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_BEGIN(op, dealloc)
     #  define CPy_TRASHCAN_END(op) Py_TRASHCAN_END
     #else