GHSA-CHGR-C6PX-7XPP PyO3 has a missing `Sync` bound on `PyCFunction::new_closure` closures

PyCFunction::newclosure and the temporary newclosurebound complement in the 0.21–0.22 series required the supplied closure to be Send + 'static but not Sync. The resulting PyCFunction is a Python callable that can be invoked from any Python thread, which means the closure may be called concurrent...