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
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