The change in gh-118157 (b2cd54a) should have also updated clear_singlephase_extension() but didn't. We fix that here. Note that clear_singlephase_extension() (AKA _PyImport_ClearExtension()) is only used in tests.
(cherry picked from commit 15d48aea02, AKA gh-121503)
Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>