templates/stage-python-tests: Set sourceFileResolver to publishCoverage

It enables to browse code coverage report at source file level in Jenkins Web UI.

Set the value to STORE_LAST_BUILD to only keep coverage report for the last
executed build job.

jobs/templates/includes/stage-python-tests.groovy.j2

@@ -20,6 +20,7 @@ stage('Python tests') {
           coberturaAdapter(path: 'coverage.xml'),
         ],
         tag: 'pytest',
+        sourceFileResolver: sourceFiles('STORE_LAST_BUILD'),
       )
 
       // JUnit report