ci: Run tox in a fixed python version

Tox doesn't need to be the same python version as the one being
tested. This works around Tox's current incompatibility with
Python 3.14b1. It will probably be reverted after that is fixed
so we no longer need to install two versions of python.

See tox-dev/tox#3523

Author: bdarnell

.github/workflows/test.yml

@@ -74,14 +74,21 @@ jobs:
       - uses: actions/setup-python@v5
         name: Install Python
         with:
-          python-version: ${{ matrix.python}}
+          # In addition to the python version under test, install one known version
+          # so we can run tox with it. This lets us avoid issues when we're testing
+          # an alpha or beta version of python that is not yet compatible with tox.
+          # TODO: do something about all of this repeated usage of python 3.11 (same
+          # the docs/lint job in tox.ini).
+          python-version: |
+            3.11
+            ${{ matrix.python}}
       - name: Install apt packages
         run: sudo apt-get update && sudo apt-get install libcurl4-openssl-dev
       - name: Install tox
-        run: python -m pip install tox -c requirements.txt
+        run: python3.11 -m pip install tox -c requirements.txt
 
       - name: Run test suite
-        run: python -m tox -e ${{ matrix.tox_env }}
+        run: tox -e ${{ matrix.tox_env }}
 
   test_win:
     # Windows tests are fairly slow, so only run one configuration here.

tox.ini

@@ -65,6 +65,7 @@ allowlist_externals = sh, env
 
 # Tox filters line-by-line based on the environment name.
 commands =
+         python --version
          # py3*: -b turns on an extra warning when calling
          # str(bytes), and -bb makes it an error.
          python -bb -m tornado.test {posargs:}