Support for top-level user configuration

[rharish101]

This should resolve #1577. At the user level, a TOML config can now be specified with the following paths (not directories, but paths to the TOML file):

• Windows: ~\.black
• Unix-like: $XDG_CONFIG_HOME/black (~/.config/black if XDG_CONFIG_HOME is not set)

These locations are searched only when no configuration in the current project directory is found. I chose these defaults because this is how flake8 and pycodestyle specify their user-level configs.

TODOs:

• Fix existing tests: Running python -m unittest results in failed tests if there's a top-level user config, but all of them pass otherwise.
• Fix black-primer: Same as above, but with black-primer.
• Add new tests.
• Add documentation: README, website, etc.

[rharish101]

Currently I've patched black-primer to use the work path as XDG_CONFIG_HOME, which basically switches off the top-level user config search. However, the Windows path cannot be configured. This means that black-primer will fail when run locally on Windows if the user has setup a top-level user config. Any advice on how I should go about fixing this?

Also, what's up with the Fuzz tests? Given that GitHub Actions wouldn't be having a top-level user config for Black, my changes shouldn't affect them. However, it seems that they're failing randomly.

[rharish101]

I've now cleaned up the git commit history for easy reviewing.

[rharish101]

Currently I've patched black-primer to use the work path as XDG_CONFIG_HOME, which basically switches off the top-level user config search. However, the Windows path cannot be configured. This means that black-primer will fail when run locally on Windows if the user has setup a top-level user config. Any advice on how I should go about fixing this?

Solved this by overriding the USERPROFILE environment variable. Python (specifically os.path and pathlib) uses this to determine the home directory on Windows. Therefore, manipulating this helped me hide the actual home directory and thus a possible top-level user config.