fix subst drive paths on Windows

[adamkerz]

On Windows, if you use subst to create a new drive from a subpath of another drive, some software resolves paths back to the previous drive and others don't.

Sublime doesn't, but git does, it appears.

Eg:
D:\development\my_repo\architecture\models.py in Sublime gets turned into C:\d_drive\development\my_repo\architecture\models.py by git. To be more correct, running git rev-parse --show-toplevel returns C:\d_drive\development\my_repo which is clearly NOT a prefix of D:\development\my_repo\architecture\models.py and this breaks the relative file path calc in the RemoteUrlCommands.

[adamkerz]

Unfortunately, os.path.realpath DIDN'T resolve the path through subst paths until python3.8, so this necessitates an "upgrade" to py3.8, meaning Sublime4+ support only. Technically, the plugin should still work fine on older versions of sublime as the os.path function still exists and the .python-version file is ignored, but the bug won't be fixed on older versions. This is the obscure end of an obscure wedge...

[ElMassimo]

Made a change so that the path calculation keeps the opened filename and only canonicalizes parent directories when the file cannot be made relative to Git’s reported root (such as a Windows subst drive).

Added regression coverage for file symlinks and directory aliases, plus a Windows Actions job that creates an actual subst drive and tests the Git root and physical-path cases.