Enable flake8 for secrets

[gruebel]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

• Enabled flake8 for dockerfile, sca_image, sca_pakage, secrets and serverless
• Enabled mypy for secrets, I had to add a couple of type:ignore because of the used detect_secrets package. The package is actually completely typed, but is missing a file called py.typed so I created a PR (add py.typed file to enable type hints for package consumers Yelp/detect-secrets#579) and hope it will be accepted and released at some point, so we can remove those comments 🚀
• Did some small performance improvements in secrets
• Removed atomicwrites dependency, because it was not used
• I also adjusted the performance test threshold a little, because Python 3.7 is currently very flaky

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules