Upgrade Python version of readthedocs to > 3.6

[astrojuanlu]

Read the Docs requires Python 3.6, which is EOL this December:

https://www.python.org/dev/peps/pep-0494/

We should upgrade the application to run on a newer version.

[humitos]

We have an opened PR at #7421 that upgrades to Ubuntu 20.04 LTS and Python 3.8. That PR also requires using Ubuntu 20.04 LTS and Python 3.8 on the production servers (done at https://github.com/readthedocs/readthedocs-ops/pull/794)

Both PRs require an upgrade probably, but I wouldn't expect too many issues. The main work will be QA on the production environment and coordinated deployment.

[astrojuanlu]

Thanks @humitos. It's not clear to me if #7421 depends on https://github.com/readthedocs/readthedocs-ops/pull/794, the other way around, or if they have to be deployed simultaneously.

[humitos]

Yes 😝

#7421 does not depend on https://github.com/readthedocs/readthedocs-ops/pull/794 to work locally. However, we cannot merge #7421 without merging https://github.com/readthedocs/readthedocs-ops/pull/794 as well because we will have Ubuntu 20.04 LTS and Py 3.8 locally but Ubuntu 18.04 LTS and Py 3.6 in production and that will produce issues.

Both PRs are required to be merged to do the deployment/release.

Does this clarify the situation?