Consider removing `six`, `past`, and `future` support

Since python2 is now no longer supported, I think it might be a good time to remove special casing of these libraries.

Why?

1. None of them are required for python3+ code
2. All their features are now (long) supported by python3+
3. Their support is clearly a hack
4. Code to support them is quite complex: https://github.com/python/mypy/blob/840a310f0b6c4a4bd7edfb141c8d1731ba4dd027/mypy/semanal.py#L1971-L2018
5. There are multiple places where six. / etc special cases are used: mypy/stubgen.py, mypy/semanal.py, mypyc/irbuild/for_helpers.py

I don't see much benefit in keeping them.

Related https://github.com/python/mypy/issues/12237