chore: Remove Python 3.7 support for pypi packaging

[lucashuy]

Which issue(s) does this change fix?

N/A

Why is this change necessary?

AWS SAM CLI is dropping support for publishing pypi packages for Python 3.7.

How does it address the issue?

Removes the classifier in the setup.py file and bumps the minimum version.

What side effects does this change have?

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

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