[3.11] gh-108822: Fix regrtest clear_caches()

[vstinner]

Python 3.11 doesn't have fractions._hash_algorithm cache.

• Issue: regrtest: compute statistics on executed tests #108822

[vstinner]

Without this change, ./python -m test test_buffer -R 3:3 fails with:

test test_buffer crashed -- Traceback (most recent call last):
File "/home/vstinner/python/3.11/Lib/test/libregrtest/runtest.py", line 391, in _runtest_env_changed_exc
_load_run_test(result, ns)
File "/home/vstinner/python/3.11/Lib/test/libregrtest/runtest.py", line 351, in _load_run_test
regrtest_runner(result, test_func, ns)
File "/home/vstinner/python/3.11/Lib/test/libregrtest/runtest.py", line 306, in regrtest_runner
refleak, test_result = dash_R(ns, result.test_name, test_func)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/vstinner/python/3.11/Lib/test/libregrtest/refleak.py", line 87, in dash_R
dash_R_cleanup(fs, ps, pic, zdc, abcs)
File "/home/vstinner/python/3.11/Lib/test/libregrtest/refleak.py", line 182, in dash_R_cleanup
clear_caches()
File "/home/vstinner/python/3.11/Lib/test/libregrtest/utils.py", line 220, in clear_caches
fractions._hash_algorithm.cache_clear()
^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'fractions' has no attribute '_hash_algorithm'
test_buffer failed (uncaught exception)