Test under python 3.10

Whilst we are still not running job-server inside docker, we need
job-runner to work under the system python. Currently, TPP is on
20.04/pyh38, andw e want to upgrade to 22.04/py3.10

job-runner does work on 3.10, as its bundled in opensafely-cli, and
that's been running on 3.10 fine for anyone on 22.04 for ages.

But this formalises it in CI.

.github/workflows/tests.yml

@@ -30,7 +30,7 @@ jobs:
         # Python 3.8 is what we currently support for running cohortextractor
         # locally, and 3.9 is what we required for databuilder so we need to make
         # sure we can run with those
-        python: [3.8, 3.9]
+        python: [3.8, 3.9, 3.10]
     runs-on: ${{ matrix.os }}
     name: Run test suite
     steps: