Python3.8 support!

[kdnakt]

What did you implement

Updated serverless.yml template for aws-python3 to use python3.8.

https://aws.amazon.com/jp/blogs/compute/python-3-8-runtime-now-available-in-aws-lambda/

Closes #XXXXX

How can we verify it

Todos

Useful Scripts

• npm run test-ci --> Run all validation checks on proposed changes
• npm run lint-updated --> Lint all the updated files
• npm run lint:fix --> Automatically fix lint problems (if possible)
• npm run prettier-check-updated --> Check if updated files adhere to Prettier config
• npm run prettify-updated --> Prettify all the updated files

• Write and run all tests
• Write documentation
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[codecov-io]

Codecov Report

Merging #6978 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #6978   +/-   ##
=======================================
  Coverage   88.42%   88.42%           
=======================================
  Files         229      229           
  Lines        8416     8416           
=======================================
  Hits         7442     7442           
  Misses        974      974

Impacted Files	Coverage Δ
lib/plugins/aws/invokeLocal/index.js	76.36% <100%> (ø)	⬆️

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[medikoo]

Thank you @kdnakt !

[medikoo]

I've added also documentation updates.