Pytest-docker

Changes:
- Drop Python3.5 support

Changes:
- Add the `docker_cleanup` fixture to allow custom cleanup actions for
docker-compose

Remove support for Python2 (couple of small changes regarding bytes/strings)
Changing package description files

migrate to setup.cfg/setup.py combination
add pylint and pycodestyle checks

Changing TravisCI configuration

add tests for Python versions 3.5 to 3.8
add PYPi deployment section

Move some things around.

Fixes 41

Update README
Update travis settings

Remove the fallback method that allowed running without Docker
Package cleanup
Fix problems with `attrs` dependency