Use the latest version of datadog-lambda

[tianchu]

What does this PR do?

• Bump the datadog-lambda version to pick up the latest improvements (specifically Disable flushing metrics in thread datadog-lambda-python#120 and Enable compressed payloads on ThreadStats object datadog-lambda-python#86).
• Update integration test to decompress payload because of Enable compressed payloads on ThreadStats object datadog-lambda-python#86
• Fix the integration test script to build the forwarder bundle against the correct python version. Previously the bundle was always built in a python 3.7 docker image, which breaks the python3.8 integration tests.

Motivation

Testing Guidelines

• The testing forwarder still works

Additional Notes

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)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)