bpo-37031: Fix PyOS_AfterFork_Child() (GH-13537)

PyOS_AfterFork_Child(): _PyInterpreterState_DeleteExceptMain() must
be called after _PyRuntimeState_ReInitThreads().

_PyRuntimeState_ReInitThreads() resets interpreters mutex after fork,
mutex used by _PyInterpreterState_DeleteExceptMain().