Diff-tool

* Refactored app
* Moved `cli` logic to its own module
* Flattened diff logic to function instead of class methods since no class was needed
* Various small improvements, added type hinting, etc
* Archiving project as I don't intend to keep maintaining it

* Drop support for Python 3.6
* Replaces the `mock` library with the builtin `unittest.mock` library
* Reworks tests to use `io.StringIO()` instead of actual temp files

* Fixed a bug when using this as a CLI tool and the tool wouldn't accept the path arguments (eg: TypeError: expected str, bytes or os.PathLike object, not TextIOWrapper)
* Refactored the code to be more clean and allow each function to only do a single thing, better variable names, default argument values, etc

* Exporting package via `__init__.py` file
* Small code cleanup

* Renamed `diff-tool` from `diff` to match published name and avoid conflict with other potential `diff` tools
* Changed all classmethods to staticmethods
* Switched from Travis CI to GitHub Actions

* Simplified unit tests