pre-commit image: Lock Cython to <3.

Needed for PyYAML<6.

images/pre-commit/Dockerfile

@@ -12,6 +12,8 @@ RUN curl -fsSL https://apt.releases.hashicorp.com/gpg | \
         python3-setuptools \
         python3-venv \
     && \
+    export PIP_CONSTRAINT=/tmp/constraint.txt && \
+    echo 'Cython < 3.0' > "$PIP_CONSTRAINT" && \
     python3 -m pip install --no-cache-dir --break-system-packages \
         docker-compose \
         pipenv \