Add CI for testing NixOS with Python 3.12

[hacklschorsch]

The already fixed ticket:4071 added Python 3.12 compatibility, but we don't test Python312 with NixOS yet.

Add a CI job testing Python 3.12.

[hacklschorsch]

Fake news - there's already jobs running (successfully) under Python 3.12:

$ rg -C2 3\.12 .github/workflows/ci.yml .circleci/config.yml

.github/workflows/ci.yml
47-        include:
48-          - os: macos-12
49:            python-version: "3.12"
50-          # We only support PyPy on Linux at the moment.
51-          - os: ubuntu-latest
--
54-            python-version: "pypy-3.9"
55-          - os: ubuntu-latest
56:            python-version: "3.12"
57-          - os: windows-latest
58:            python-version: "3.12"
59-
60-    steps:
--
171-        include:
172-          - os: ubuntu-20.04
173:            python-version: "3.12"
174-            force-foolscap: true
175-    steps:

.circleci/config.yml
7-# source of our dots), but switching to that is going to be a bigger refactor:
8-#
9:#   https://discuss.circleci.com/t/v2-1-job-name-validation/31123
10-#   https://circleci.com/docs/2.0/reusing-config/
11-#
--
131-              pythonVersion:
132-                - "3.9"
133:                - "3.12"
134-
135-      - "integration":

[hacklschorsch]

Oh man, CI with current Nixpkgs is breaking in really stupid ways, none of which have anything to do with Tahoe-LAFS. This isn't worth the time.