Drop support for Python 3.6

[swcurran]

It’s now time to drop support for Python 3.6. What needs to be done to make that happen? I’m guessing the list is something like:

• Change the requirements for ACA-Py to use a minimum of 3.7
• Update the documentation (Readme, elsewhere?)
• Stop running the automated 3.6 tests. If they are the per PR tests, change those to use Python 3.9 (right?)
• Update any container image publishing to use Python 3.9.

FYI BC Gov has updated all deployments from Python 3.6 to 3.9 and there were no issues identified.

Adjustments/additions @WadeBarnes @dbluhm ?

[WadeBarnes]

I think we should go straight to using a minimum of Python 3.9. All of the tests are passing for that version and we're already building and publishing images for py3.9 here https://github.com/hyperledger/aries-cloudagent-python/pkgs/container/aries-cloudagent-python. BC Gov, as @swcurran pointed out, is using these images to upgrade all services and we've had great success using the py3.9 images as drop-in replacement/upgrade for all previous versions. Moving to py3.9 as the minimum means we just need to drop the support for py3.6 and py3.7 in a few GitHub workflows from a CI/CD perspective.

So:

• Change the requirements for ACA-Py to use a minimum of Python 3.9
• Update the documentation (Readme, elsewhere?)
• Drop support for 3.6 and 3.7 in all automated tests. Switch to using 3.9 where needed (though I think it's included in most workflows already).
• Stop publishing container images containing Python 3.6. (We're already publishing images for Python 3.9)

@swcurran, as a separate topic we need to discuss what to do with bcgovimages/aries-cloudagent due to it's 50M+ download profile.

[swcurran]

I think we need to transition the bcgovimages/aries-cloudagent as well, but open to suggestions on the best way. I guess since the image is a replacement, we should be able to just move the latest tag to a Python 3.9 one?

[WadeBarnes]

Could be as simple as relabeling/retagging the image and pushing it to the exiting repo, and updating the documentation to indicate it's acting as a mirror of sorts.