Custom Lambda runtime, how to get python3 available in layer?

Hi,

I want to use a custom runtime for my Lamba function and implement it in S3. To start off I was able to get the example from https://docs.aws.amazon.com/lambda/latest/dg/runtimes-walkthrough.html working in serverless using the following snippets in my serverless.yml:

layers:
  scientistRuntime:
    path: custom_runtime_layer
    name: ${self:provider.stage}-scientistRuntime # optional, Deployed Lambda layer name
    description: Custom runtime for Scientst
    compatibleRuntimes:
      - python3.7

functions:
  test_layer:
    handler: scientist.lambda_handler
    runtime: provided
    layers:
      - { Ref: ScientistRuntimeLambdaLayer }
    events:
      - http:
          path: test
          method: any
          cors: true
      - http:
          path: test/{experiment}
          method: any
          cors: true

However, it looks like python3 is not available, just python 2.7.16 (added python --version to echo python version to logs) is available. What do I need to do to be able to use python3.7? Adding it as compatibleRuntimes does not seem to be enough.

Thanks,
Gero