Remote cache doesn't work in Windows due to DOS style path

[LAWArthur]

In Windows, remote cache could never be fetched due to the DOS style path which uses backslashes.

For example, tracing Mathlib results in https://dl.fbaipublicfiles.com/lean-dojo\leanprover-community-mathlib4-29dcec074de168ac2bf835a77ef68bbe069194c5.tar.gz at cache.py:60, which urllib cannot recognize as a valid url due to the backslash.

A possible fix could be using pathlib instead of os.path.

[LAWArthur]

Since there are a lot more problems with the path system, I see that maybe this project is not ready for cross-platform usage. But I hope you could seriously take this into consideration.