ireally drop python<=3.7 support

[kloczek]

Description of changes

Filter all code over pyupgracde --py38-plus.

Description of how you validated changes

After patch the code pytest has been executed.

Checklist

• [x] Adheres to the development guidelines
• [ ] Add/update transform tests
  • [ ] Using correct values
  • [ ] Using wrong values
• [ ] Add/update integration tests

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

[seshubaws]

Hi @kloczek thank you for opening this PR! Could you resolve the conflicts in the two files aws_serverless_api.py and aws_serverless_function.py so we can run the tests?