gh-actions: pin to tox 3 for now

Author: miri64

.github/workflows/tox.yml

@@ -36,7 +36,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install tox
+        python -m pip install "tox<4"
         sudo apt-get update
         sudo apt-get install -y tmux
     - name: Test with tox