Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.
Similar to other linters/fixers, by default change to the directory of
the file being fixed before invoking `black`, which allows the tool to
read project-specific configuration (pyproject.toml)
Fixes#2218
