Improve error handling for Zyte API

[Gallaecio]

• Treat 429 and 503 responses with a Zyte-Error header the same as noslaves 503 responses from Smart Proxy Manager.
• Treat 520 and 521 responses with a Zyte-Error header the same as banned 503 responses from Smart Proxy Manager.
• CI stuff:
  • Move bandit to pre-commit as part of silencing errors about assert use
  • Add a mypy job, since mypy is the reason why we allow asserts in code
  • Remove Python 3.5 from CI for technical reasons; we continue to test 3.4, though.
  • Use caching to speed up Python < 3.8 jobs.

cc @janin @neoyogi @BurnzZ @PyExplorer