Support for current versions of Python

[jaredswarren]

python 3.6 is end of life. To continue supporting python, support for more recent versions should be added.

[jem473]

It should be noted that you can still use this with more recent versions of Python (or any runtime) by specifying the ARN of the layer. There is a pull request to mark more recent versions as compatible, but it's almost two years old at this point.