Update locustio/locust Docker image to Python 3.9.6

[istepanko]

Python 3.10 will be released in 2 months, ideally package should not be more than 1 release cycle behind.

Describe the solution you'd like

Update Python version to 3.9.6

Additional context

I am using Locust + Python 3.9.x for already 6 months and 3.9.6 for a month - no new bugs found.

[cyberw]

I dont think we should upgrade just yet. 3.8 is still the most common version in the wild, and 3.9 hasnt really taken off yet (only ~6% of locust pypi downloads are 3.9, both 3.7 and 3.8 are much more common). Maybe in 6 months or so.

[StevenBorg]

One note, that may be relevant. running Snyk against the latest locustio/locust docker instance, shows 361 vulnerabilities.

Repro steps
docker pull locustio/locust:latest
docker scan --accept-license locustio/locust

Notes from scan and recommended steps (not including the long list of security vulnerabilities):
Package manager: deb
Project name: docker-image|locustio/locust
Docker image: locustio/locust
Platform: linux/amd64
Base image: python:3.8.11-buster

Tested 431 dependencies for known vulnerabilities, found 361 vulnerabilities.

Your base image is out of date

1. Pull the latest version of your base image by running 'docker pull python:3.8.11-buster'
2. Rebuild your local image

For more free scans that keep your images secure, sign up to Snyk at https://dockr.ly/3ePqVcp

[cyberw]

I think these are separate issues, but I'm starting to think 3.9 is a good idea now (it has been a while longer, and I realized that it doesnt really matter, because probably nobody needs it to be running specifically 3.8).

3.8 is always latest 3.8.x, so it should be the same though? And how did you end up with python:3.8.11-buster?

[cyberw]

Anyway, PR for 3.9 is welcome.

[jaywgraves]

OK to close?
seems to be fixed in 08c3c11