Fix spyx_tmp() cleanup.

For some reason, a new behaviour of python 3.13 [0] causes the
`TemporaryDirectory()` in `sage.misc.temporary_file.spyx_tmp()`
to be deleted on child exit, which causes trouble with parallel
doctesting [1].

We rewrite `spyx_tmp()` using `tmp_dir()`, which doesn't have this
problem, see [2].

[0] python/cpython#114279
[1] sagemath#39188 (comment)
[2] sagemath#39188 (comment)

src/sage/misc/temporary_file.py

@@ -533,14 +533,11 @@ def spyx_tmp() -> str:
     We cache the result of this function "by hand" so that the same
     temporary directory will always be returned. A function is used to
     delay creating a directory until (if) it is needed. The temporary
-    directory is removed when sage terminates by way of an atexit
-    hook.
+    directory is automatically removed when sage terminates.
     """
     global _spyx_tmp
     if _spyx_tmp:
         return _spyx_tmp
 
-    d = tempfile.TemporaryDirectory()
-    _spyx_tmp = os.path.join(d.name, 'spyx')
-    atexit.register(lambda: d.cleanup())
+    _spyx_tmp = tmp_dir(name='spyx_')
     return _spyx_tmp