python/cpython#113829 changed `os.path.isabs` on Windows to not consider a path starting a single backslash to be an absolute path.
We do a lot of naive `posixpath` to `ntpath` conversions, and it broke DVC in multiple places. DVC is likely broken in many other places, and these bugs are difficult to identify.
In this commit, I have tried to fix in the places where the tests failed. And mostly by trying to imitate pre-3.13 behaviour.
Some of the changes may not be correct but keeps the old behaviour in place.
Also re-enabled the CI for all Python versions supported for Windows.