perform type testing with `mypy`

[jGaboardi]

• add config to pyproject.toml
  • example
• add config to .pre-commit-config.yaml
  • example
• either add: not needed since pre-commit is enabled
  • a stand-alone action - as in momepy - or
  • adapt testing.yml to run mypy only on ubuntu-latest
• xref:
  • add type hints #55
  • interest in modernizing? #20