bpo-46581: Propagate private vars via _GenericAlias.copy_with (GH-31061)

bpo-46581: Propagate private vars via _GenericAlias.copy_with (GH-31061)

GH-26091 added the _typevar_types and _paramspec_tvars instance
variables to _GenericAlias. However, they were not propagated
consistently. This commit addresses the most prominent deficiency
identified in bpo-46581 (namely their absence from
_GenericAlias.copy_with), but there could be others.

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>