Add Python 3.10 to the Docker image once it's GA

[leahecole]

Once Python 3.10 is released, we need to update to install it

python-docs-samples/.kokoro/docker/Dockerfile

Line 120 in 9698be4

&& for PYTHON_VERSION in 2.7.18 3.6.13 3.7.10 3.8.8 3.9.2; do \

[leahecole]

And once that change is put through, message the DPEs to let them know on the thread that I made 🐍

[leahecole]

I did run some tests with 3.10 in the Docker image against my composer samples and they failed because numpy doesn't support 3.10 yet. When I get some spare time (ha) I will run against all samples to see how many it fails on and if it's only a handful, to enable it but expect failures (and not require the check), or, if it's failing on the majority of samples, wait to enable it a bit longer.