Boost `ltitop` quality

[hidmic]

Rationale

As of today, ltitop is still pretty much a proof-of-concept. Thus, its code needs polishing, test coverage must be increased, and a continuous integration process be put in place to ensure it stays healthy over time.

Definition of Done

• [x] Add pre-commit hooks
  • [x] Reformat sources using black and blacken-docs
  • [x] Reformat sources susing isort.
  • [x] Lint sources using flake8.
  • [x] Statically check sources using mypy.
  • [x] Lint tox.ini using tox-ini-fmt
• [x] Test for Python 3.5+ using tox
• [ ] Measure coverage using pytest-cov
• [x] Configure Github Actions CI
  • [x] Test job to ensure all unit tests pass
  • [x] Lint job to ensure all linters pass
• [ ] Test all public APIs (~80% coverage)
• [ ] Type annotate all public APIs.

Additional considerations

Consider Codecov or Coveralls for coverage tracking.