fix(pubsublite): ignore python 3.10

[anguillanneuf]

Fixes #8275
Fixes #8276

Flaky when running on multiple python versions. Really only one version is sufficient.

[dandhlee]

LGTM.

[anguillanneuf]

@dandhlee This is a weird error for the test:

RROR: This script does not work on Python 3.6 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.6/get-pip.py instead.
The command '/bin/sh -c wget --no-check-certificate -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' && python3.6 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" && pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" && [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ]' returned a non-zero code: 1

https://source.cloud.google.com/results/invocations/df2528fc-34a2-4e76-9143-0d685f6a6a97/targets/cloud-devrel%2Fpython-docs-samples%2Fpython3.9%2Fpresubmit/log

Have we misconfigured something?

---

And the error went away.

[dandhlee]

Kokoro should never have that much logs being generated from Docker build, it does happen from time to time with system errors. Since it worked I wouldn't worry, usually it's a thing on the Kokoro side (we did have Kokoro team tell us they had an update this morning and things have been slow today)