Set min supported Python version to 3.8 (#93155)
Also, grep for `if sys.version_info .cond. (3, 8)` and replaces them with appropriate action.
This is a last in a series of PRs that moved CI/CD away from testing PyTorch behavior against Python-3.7.
Fixes https://github.com/pytorch/pytorch/issues/80513
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93155
Approved by: https://github.com/huydhn
