Introduce isort and gitlab-ci-linter pre-commit hooks
### Isort Import sorting - https://pycqa.github.io/isort/ - https://github.com/pycqa/isort ### Todos - [x] Add `isort` as dev dependency - [x] Sort imports in `silkaj` and `tests` folders - [x] Add `pre-commit` hook - [x] Add `isort` job: - [x] Install `isort` on Py3.8 image ##### `gitlab-ci` - https://gitlab.com/devopshq/gitlab-ci-linter - https://gitlab.com/smop/pre-commit-hooks
issue