windows: continue looking at `%HOME%` for user config files with py3.8+ The `%HOME%` variable is explicitly called out in `hg help config` as a location that is consulted when reading user files, but python stopped looking at it when expanding '~' in py3.8+.[1] Restore that old functionality by copying in the old implementation (and simplifying it to just use bytes). It could be simplfied further, since only '~' is passed, but I'm not sure yet if we need to make this a generic utility function on Windows. There are other uses of `os.path.expanduser()`, but this is the only case I know of that documents `%HOME%` usage. (The reason for removing it was that it typically isn't set, but it actually is set in MSYS and PowerShell, and `%HOME%` and `%USERPROFILE%` are different in MSYS. I could be convinced to just replace all uses with this as a general utility, so we don't have to think too hard about BC.) [1] https://bugs.python.org/issue36264 Differential Revision: https://phab.mercurial-scm.org/D9559