Use built-in importlib.resources on Python 3.9+ (#7042)

Use built-in `importlib.resources` module rather than the external
`importlib_resources` package on Python 3.9 and newer.  The latter
is only intended as a backport for old Python versions, and since modern
Linux distributions may no longer support such old Python versions,
they also no longer provide importlib_resources (this is the case
on Gentoo).

scripts/update_api.py

@@ -1831,7 +1831,10 @@ def write_core_py_preamble(core_py):
 import sys, os
 import contextlib
 import ctypes
-import importlib_resources
+if sys.version_info >= (3, 9):
+    import importlib.resources as importlib_resources
+else:
+    import importlib_resources
 from .z3types import *
 from .z3consts import *