Drop support for Python 3.7

[aahung]

Why

• Python 3.7 is EOL: https://devguide.python.org/versions/
• SAM CLI stopped publishing 3.7 to pypi: chore: Remove Python 3.7 support for pypi packaging aws-sam-cli#6119
• Features like TypedDict and Literal are native in 3.8 and we can drop from importing typing_extensions

[xazhao]

Thanks for the issue @aahung. We will be starting to drop the suppor for Python3.7

[ssenchenko]

PR merged #3403