tox: Force urllib dependency to a compatible version with requests

This should fix the current "upload to pypi" build step.

Refs. swh/infra/sysadm-environment#4870

tox/Dockerfile

@@ -9,6 +9,9 @@ RUN mkdir -p /home/jenkins/.cache/pip
 
 RUN --mount=type=cache,id=pip-cache,target=/home/jenkins/.cache/pip,uid=115,gid=120 \
   python3 -m pip install --user --upgrade --no-warn-script-location \
-  pip setuptools wheel twine tox \
+  pip setuptools wheel \
+    twine \
+    tox \
+    "urllib3<1.27,>=1.21.1" \
   Radon pylint coverage codespell pyarcanist pre-commit && \
   python3 -m virtualenv --upgrade-embed-wheels