The prior attempt to resolve#420 and #431 relied on `appdirs` to
determine the appropriate directories into which to install files.
Previously, `setup.py` dynamically attempted to install `appdirs` via
`pip` during installation if necessary.
This attempt to failed on multiple platforms, however, due to
backwards-incompatible `pip` interfaces.
As a workaround, I have now directly included `appdirs` (a small module)
within `cheat` itself. This approach is explicitly supported per the
`appdirs` documentation:
https://pypi.org/project/appdirs/
- Added `ci/lint.sh`, which uses `flake8` to lint the relevant files
- Made changes to appease the linter.
- Bugfix in `cheat/configuration` (missing dependency)
