Notable changes:

1. Dropping support for Python versions older than 3.7
2. Vendoring pip to get better reusable utilities since pip._internal doesn't look friendly
3. Fix dirty patches such as: https://github.com/damnever/pigar/blob/a608132eb80e8ff4b3b98848c70555f648adccfa/pigar/core.py#L31
4. MAYBE custom configuration file to work around damnever/pigar#32
5. MAYBE rework the command line interface