Add UnpackType type and basic semanal (#12367)
Adds very basic support for Unpack expressions (where Unpack
is either from typing or typing_extensions). Many of the visitors
implementations are not implemented yet.
The only supported feature we have so far is validating that the
argument to UnpackType actually makes sense. Currently we don't
support unions, though. Since TypeVarTuple isn't yet implemented,
we don't support that from UnpackType either.
We don't want anyone to start using variadic generics at all until
we have real support for it. We also don't want to implement it in
one huge PR. So mypy is learning a new flag called
--enable-incomplete-features which we can enable by default in any
testsuite that needs to use incomplete features, as well as pass on
the command line for debugging, but it is undocumented and suppressed
from the help output so nobody should find it.
