* Bump v3.0.0

* Adding Slack invitation link

* Move tests to tests/ directory
* Remove unused `tox.ini` file
* Add flake8 compliance
* Add `lint` target to `tests` target in the Makefile

 Remove unnecessary `run_tests.sh` and `run_linter.sh` scripts, and move logic directly to Makefile. (#327)

Add base layer with installing requirements to shorten build time.
Add .dockerignore to reduce amount of files to copy.
Remove unnecessary `run_docker.sh` script.