[BE] Remove dependency on `six` and `future` (#94709)
Remove the Python 2 and 3 compatibility library [six](https://pypi.org/project/six) and [future](https://pypi.org/project/future) and `torch._six`. We only support Python 3.8+ now. It's time to retire them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94709
Approved by: https://github.com/malfet, https://github.com/Skylion007
