[Bug] Fix packaging approach

[D3vil0p3r]

setup.cfg calls requirements.txt that mixes both runtime and build dependencies.

This approach must be improved because there is no need to have build dependencies installed at runtime. On the packaged version of the tool this approach generates issues.

You can solve this in two manners:

1. Create two separated requirements files: one for build deps requirements-dev.txt (pytest and flake8) and another one for runtime deps requirements.txt (the remaining deps in requirements.txt) and change setup.cfg according to this doc by specifying the requirement file for the build system and the one for the runtime system.
2. Better solution: to drop the obsolete setup.py anyway and migrate to pyproject.toml that offers more flexibility cf. https://python-poetry.org/docs/master/managing-dependencies/

Ref: BlackArch/blackarch#3940