feat: Add support for Python 3.9 runtime

[eeroniemi]

Description

Add support for Python 3.9 runtime

Motivation and Context

Motivation was to get serverless-offline to support latest stable Python version (3.9).

Fixes issue: #1266

How Has This Been Tested?

I cloned serverless-offline repo, built it, and included it in my Serverless Framework project. Before my patch, when trying to use Python 3.9 I got error:

offline: Warning: found unsupported runtime 'python3.9' for function 'foo'
offline: Failure: Unsupported runtime

After this change there was no error and everything worked as expected.

[pgrzesik]

Thanks @eeroniemi 🙇 I think we should also still include message similar to this one:

serverless-offline/src/lambda/handler-runner/HandlerRunner.js

Lines 43 to 48 in 6eb15d7

	if (runtime === 'nodejs14.x') {
	logWarning(
	'"nodejs14.x" runtime is not supported with docker. See https://github.com/lambci/docker-lambda/issues/329',
	)
	throw new Error('Unsupported runtime')
	}

as the python3.9 is not supported in docker at the moment

[eeroniemi]

Hi @pgrzesik. I added check as you suggested. Now if I try run serverless-offline with useDocker: true configuration option and python3.9 runtime, I get error:

offline: "python3.9" runtime is not supported with docker.
offline: Failure: Unsupported runtime

[pgrzesik]

Looks good, thank you @eeroniemi 🙇

[pgrzesik]

FYI: This change has been released with 8.1.0 release today