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.
0324d1740e