* Invoke self-install via PEP517 in the CI
* Add tox envs for building dists via PEP 517
* Add linting dists via tox to GHA
* Upgrade `master` to `develop` in `setup.py`
* Simplify `python_requires` in `setup.py`
* Convert dynamic `setup.py` into static `setup.cfg`
* Go flagless to allow custom user defined flags. Fixes#301
* Add --cache-dir flag for cache plugin (when used with on-disk store)
* Enable discovery of flags from external plugins, example those that reside outside of proxy.py package and loaded on demand. This also allows external flags to surface in --help section
* Define --filtered-client-ips flag for FilterByClientIpPlugin
Sviatoslav Sydorenko