Add tests with pylint and pycodestyle

.gitlab-ci.yml

.pylintrc

requirements-dev.txt

requirements.txt

src/briar-gtk.in

src/briar/api/api.py

src/briar/api/model.py

src/briar/api/models/contacts.py

src/briar/api/models/private_chat.py

src/briar/api/models/socket_listener.py

src/briar/gtk/application.py

src/briar/gtk/container.py

src/briar/gtk/containers/chat.py

src/briar/gtk/containers/main.py

src/briar/gtk/containers/startup.py

src/briar/gtk/define.py

src/briar/gtk/toolbar.py

src/briar/gtk/window.py

src/meson_post_install.py

tools/run-tests.sh