Fix tox -e twine

Author: twm

.github/workflows/ci.yaml

@@ -27,7 +27,7 @@ jobs:
           ${{ runner.os }}-pip-lint-
           ${{ runner.os }}-pip-
 
-    - run: python -m pip install tox
+    - run: python -m pip install 'tox<4'
 
     - run: tox -q -p all -e flake8,towncrier,twine,check-manifest
 

CONTRIBUTING.rst

@@ -1,7 +1,7 @@
 Developing
 ==========
 
-This project uses `Tox <https://tox.readthedocs.io/en/latest/config.html>`_ to manage virtual environments.
+This project uses `Tox 3 <https://tox.readthedocs.io/en/latest/config.html>`_ to manage virtual environments.
 
 To run the tests::