Fix regex pattern in .gitlab-ci.yml

The regular expression pattern for the coverage line in the .gitlab-ci.yml file was not properly formatted. It was causing issues in the CI pipeline as it couldn't correctly parse the coverage percentage. This commit wraps the expression in single quotes to correct this issue.

.gitlab-ci.yml

@@ -5,7 +5,7 @@ pytest:
     - apt-get install -y python3-dev python3-pip graphviz graphviz-dev
     - pip install '.[test]'
     - pytest --cov --cov-report xml:coverage.xml
-  coverage: TOTAL.*\s+(\d+%)$
+  coverage: 'TOTAL.*\s+(\d+%)$'
   artifacts:
     reports:
       coverage_report: