Support decrypting API keys encrypted with an encryption context

[nhinsch]

What does this PR do?

When decrypting the API key using KMS, try first with the encryption context added by the Lambda console UI, then try without.

Bonus: Updates black (the code formatter) to v21.

Motivation

The Lambda console UI changed the way it encrypts environment variables. The current behavior is to encrypt environment variables using the function name as an encryption context. Previously, the behavior was to encrypt environment variables without an encryption context. The Lambda Extension currently only supports API keys encrypted using the previous behavior. We need to support both, as supplying the incorrect encryption context will cause decryption to fail.

Testing Guidelines

I added unit test coverage for this function and tested manually.

Types of Changes

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

Check all that apply

• 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)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[DarcyRaynerDD]

LGTM