[Windows] Ensure relpath is executed between paths in the same drive

[adferrand]

On Windows you can have several drives (C:, D:, ...), that is the roughly (really roughly) equivalent of mount points, since each drive is usually associated to a specific physical partition.

So you can have paths like C:\one\path, D:\another\path.

In parallel, os.path.relpath(path, start='.') calculates the relative path between the given path and a start path (current directory if not provided). In recent versions of Python, os.path.relpath will fail if path and start are not on the same drive, because a relative path between two paths like C:\one\path, D:\another\path is not possible.

In saw unit tests failing because of this in two locations. This occurs when the certbot codebase that is tested is on a given drive (like D:) while the default temporary directory used by tempfile is on another drive (most of the time located in C: drive).

This PR fixes that.

[bmw]

LGTM!