Install isort in the container environments

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

dev/containers/fedora-rpms-py3

@@ -14,6 +14,7 @@ RUN dnf -y --enablerepo=updates-testing install \
     python3-docutils \
     python3-black \
     python3-flake8 \
+    python3-isort \
     python3-pytest-xdist \
     python3-flask-oidc \
     python3-cchardet \