bpo-43224: Implement PEP 646 changes to genericaliasobject.c (GH-31019)
Specifically, prepare for starring of tuples via a new genericalias iter type. GenericAlias also partially supports the iterator protocol after this change.
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com> Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com> Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>