Add GitHub Workflow

[eyllanesc]

• [x] Add GitHub workflow.
• [x] Add tox.
• [x] Fixes flake8 errors.
  • [x] Fixes #188
• [x] Add black.
• [x] Add support python 2.7, 3.6, 3.7, 3.8, 3.9 and 3.10.
  • [x] Fixes the requirements in setup.py
• [x] Fixes #203

You have to add the PyPi token to secrets.PYPI_TOKEN (See https://docs.github.com/en/actions/security-guides/encrypted-secrets#creating-encrypted-secrets-for-an-environment).