✔ CircleCI: Explicitly install binary opencv-python-headless (dep of OCR-D?) to avoid compilation

.circleci/config.yml

@@ -10,6 +10,7 @@ jobs:
     steps:
       - checkout
       - run: pip3 install --upgrade pip
+      - run: if python3 --version | grep -q "Python 3.6"; then pip install --prefer-binary -U opencv-python-headless numpy; fi  # to avoid compilation
       - run: pip3 install -r requirements.txt
       - run: pip3 install pytest
       - run: pytest