[modules/lambda_splunk_forwarder/lamdba.tf] Incorrect version of Python

[OllieJC]

The Lambda Python is written in Python 3.6 due to library issues (such as re) in Python 3.7.
Our code is working and tested with 3.6: https://github.com/alphagov/cyber-cloudwatch-fluentd-to-hec

Can the terraform be set to 3.6:
https://github.com/alphagov/gsp-terraform-ignition/blob/a1987d850a834d2df2bf74dd1a068df1529cc327/modules/lambda_splunk_forwarder/lambda.tf#L8