For 3.7 and 3.8, I have just tried with this command to find leaks in the test_codec* but nothing :/

./python -m test -v -l -R 6:6 -u all \
    test_codecencodings_iso2022 \
    test_codecs \
    test_codecmaps_hk test_codecmaps_tw \
    test_codecencodings_tw \
    test_codecencodings_cn \
    test_codeccallbacks \
    test_codecencodings_hk \
    test_codecencodings_jp \
    test_codecencodings_kr \
    test_codecmaps_jp \
    test_codecmaps_cn \
    test_codecmaps_kr

If you have another technic for the detection of leaks, please inform me but I think this issue could be closed maybe we have fixed the issue with time.