text: Title: [FEATURE] Run Python 3.9 builds
  Body: - Add `39` to `envlist` in `tox.ini`
  - Add `39` to `depends` of the `coverage-report` job in `tox.ini`
  - Add `Programming Language :: Python :: 3.9` to the classifiers list in `setup.py`
  - Add Python 3.9 job in `.github/workflows/build.yml` similarly to other builds