circleci: use Agg backend

Author: nschloe

.circleci/config.yml

@@ -54,7 +54,7 @@ jobs:
       - run: pylint matplotlib2tikz/
       - run: pylint test/*.py
       # The actual test
-      - run: cd test/ && pytest --cov matplotlib2tikz
+      - run: cd test/ && MPLBACKEND=Agg pytest --cov matplotlib2tikz
       # submit to codecov
       - run: apt-get install -y curl
       - run: bash <(curl -s https://codecov.io/bash)