refactor: setup.cfg and dev requirements (#504)

* refactor: use setup.cfg instead

* refactor: use dev extra

* fix: restore dev requiremetns

.circleci/prepare.sh

@@ -4,6 +4,6 @@ set -o xtrace
 $PYTHON --version
 $PYTHON -m pip --version
 $PYTHON -m virtualenv -p $PYTHON venv
-venv/bin/python -m pip install -r requirements-dev.txt
+venv/bin/python -m pip install -e ".[dev]"
 venv/bin/python -m pip freeze
 venv/bin/python --version