Enable mypy --strict for future code

[xazhao]

Description of changes

Use https://pypi.org/project/mypy-clean-slate/ to add ignore to all existing method. Enable mypy --strict so any future development must be typed.

Description of how you validated changes

Trying to create a function without type and make pr failed.
Trying to cast return to Any and make pr also failed.

Checklist

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• Update documentation
• Verify transformed template deploys and application functions as expected
• Do these changes include any template validations?
  • Did the newly validated properties support intrinsics prior to adding the validations? (If unsure, please review Intrinsic Functions before proceeding).
    • Does the pull request ensure that intrinsics remain functional with the new validations?

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

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

[hoffa]

Nice!

[aahung]

thx for marking EVERY SINGLE ONE OF THEM 👍