Upgrade to python 3.10 by default (#13234)

[melroy89]

See title.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

Signed-off-by: Melroy van den Berg

Regards,
Melroy van den Berg

[DMRobertson]

There are a bunch of unrelated formatting changes here which I'd prefer to be a separate change (if included at all).

We wouldn't be willing to accept this until CPython 3.11 releases, which is expected in October 2022 (per the policy described here #8674 (comment)).

In the meantime, you can build a version of this image yourself as described here.

synapse/docker/Dockerfile

Line 15 in 7ad995a

# DOCKER_BUILDKIT=1 docker build -f docker/Dockerfile --build-arg PYTHON_VERSION=3.10 .

I appreciate that's not as convenient (and won't apply to new releases) but it might give you a way forward for now.

[clokep]

I've removed this from the review queue per @DMRobertson's comments.

[richvdh]

let's close this for now - @Danger89 if you'd like to open a new PR once 3.11 is out, that would be grand!

[DMRobertson]

I reopened this now that 3.11 is released, but it looks like there's a merge conflict here. I'll open a new PR with this change.