Add support for Python 3.9, remove region validation

[nhinsch]

What does this PR do?

• Add support for the Python 3.9 runtime
• Remove support for Node 8.10 runtime. This is not a breaking change because it has not been possible to update or create a Lambda function using this runtime since March 6, 2020.
• Remove region validation. The list of regions we validated against was out of date (it did not include me-south-1, af-south-1, or eu-south-1). It's difficult to keep this list up to date, and we don't do this type of validation in our other deployment tools.

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog