Make use of walrus operator

Co-authored-by: Adam Turner <[email protected]>

Author: stephenfin

sphinx/ext/coverage.py

@@ -136,8 +136,7 @@ def _determine_py_coverage_modules(
             continue
 
     # if there are additional modules then we warn (but still scan)
-    additional_modules = set(seen_modules) - modules
-    if additional_modules:
+    if additional_modules := set(seen_modules) - modules:
         logger.warning(
             __('the following modules are documented but were not specified '
                'in coverage_modules: %s'),