Add a CI pipeline to lint and test osc dput

• Add flake8/isort settings, make sure the code is compliant.
• Add a pipeline which:
  • Lints the code
  • Attempts to build a package.
  • Attempts to actually use the built package:
    • Install osc and osc dput, verify both can run
    • Try to upload something to an OBS instance
    • Try to upload lots of tricky edge cases

(cc @em, @smcv)