Update python version to 3.8

[iwootten]

What is the context of this PR?

Updates runner to python 3.8, as per all validator and schema repos.

Our main blocker to doing this was that gevent did not have support for python 3.8 and took a large amount of time to install. Version 1.5 of gevent was recently released, providing this support.

Additionally, this PR:

• removes the pinned version of grpcio, which was set when we were exploring different approaches to including new relic within runner. There has since been multiple releases of it.
• pins the version of pylint, due to errors incorrectly raised on dynamic wtform classes

How to review

Check the tests pass. Check if the gevent installation takes abnormal length of time to install.

[pricem14pc]

Successfully run unit tests, functional tests and docker build
gevent installation successful with no abnormal observations
Successfully deployed to Kubernetes with Helm and tested in GCP
Currently documenting successful performance tests

[iwootten]

I think it worth bumping the versions in this given the PR is now almost a month old. As I mentioned gevent has had two releases in the interim.

[pricem14pc]

Added a don't merge until the performance investigation documentation has been completed and reviewed.