Pin tox version in CI (#9460)

THe most recent release tox, 4.4.0, changed the behavior of how tox
interacts with constraints files. It is raising an error because it
doesn't like the format of our constraints file in qiskit. This commit
works around this by pinning our tox version. Given that we're preparing
to release 0.23.0 pinning is the fastest solution in the short term, we
can adjust things after the release is out.

.azure/docs-linux.yml

@@ -21,7 +21,7 @@ jobs:
       - bash: |
           set -e
           python -m pip install --upgrade pip setuptools wheel
-          pip install -U tox
+          pip install -U "tox<4.4.0"
           sudo apt-get update
           sudo apt-get install -y graphviz
         displayName: 'Install dependencies'