Introduce mypy, pylint, isort, gitlab-ci pre-commit hooks

Pre-commit hooks

mypy

• Introduce mypy pre-commit hook

pylint

• Introduce pylint pre-commit hook
  • #135 (closed): Move pylint conf to pyproject.toml

isort

• Introduce pre-commit hook
• Sort imports
• Introduce isort job
  • Install isort on the 3.8 image

gitlab-ci

Doing requests on a GitLab instance

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

• Update documentation