Introduce isort and gitlab-ci-linter pre-commit hooks

Isort

Import sorting

• https://pycqa.github.io/isort/
• https://github.com/pycqa/isort

Todos

• Add isort as dev dependency
• Sort imports in silkaj and tests folders
• Add pre-commit hook
• Add isort job:
  • Install isort on Py3.8 image

gitlab-ci

• https://gitlab.com/devopshq/gitlab-ci-linter
• https://gitlab.com/smop/pre-commit-hooks