Better handling of Any/object in variadic generics (#18643)
Fixes https://github.com/python/mypy/issues/18407
Fixes https://github.com/python/mypy/issues/17184
Fixes https://github.com/python/mypy/issues/16567
There are three things here:
* Allow erased variadic callables with non-empty prefix to be supertypes
of the non-erased ones. This relaxes a bit callable subtyping in
general, but IMO this makes sense, people who want to be strict should
simply use `*args: object` instead. An alternative would be to track
erased variadic callables explicitly, which is ugly and fragile.
* Add important missing case in `subtypes.py` for `*Ts` w.r.t.
`Any`/`object` that handles similar situations for variadic instances
and tuples (here however there is nothing special about `Any` vs
`object`).
* I also fix inconsistency in join uncovered by the above two.
The changes in `expandtype.py` are no-op, I just noticed potential
danger while playing with this, so wanted to highlight it with comments
for the future.
