gh-118362: Fix thread safety around lookups from the type cache in the face of concur...

gh-118362: Fix thread safety around lookups from the type cache in the face of concurrent mutators (#118454)

Add _PyType_LookupRef and use incref before setting attribute on type
Makes setting an attribute on a class and signaling type modified atomic
Avoid adding re-entrancy exposing the type cache in an inconsistent state by decrefing after type is updated