ci: avoid heredoc in coverage summary

Jenkinsfile

@@ -109,14 +109,7 @@ python -m slipcover \
   --source ariadne \
   --fail-under "${COVERAGE_MIN}" \
   -m pytest -ra -vv --durations=20 --junitxml "${JUNIT_XML}"
-python - <<'PY'
-import json
-with open("build/coverage.json", "r", encoding="utf-8") as handle:
-    payload = json.load(handle)
-summary = payload.get("summary") or {}
-percent = summary.get("percent_covered")
-print(f"Coverage summary: {percent:.2f}%" if percent is not None else "Coverage summary unavailable")
-PY
+python -c "import json; payload=json.load(open('build/coverage.json', encoding='utf-8')); percent=(payload.get('summary') or {}).get('percent_covered'); print(f'Coverage summary: {percent:.2f}%' if percent is not None else 'Coverage summary unavailable')"
 '''.stripIndent())
         }
       }