Python code modernization, 1/n (#1141)

* Remove unicode text prefixes.

* Replace str.format() uses with f-strings.

* Replace % with f-strings, and do some cleanup.

* Fix wrong variable.

* Avoid unnecessary string conversion.

tests/unit/plugins/test_support/docker_image_archive_stubbing.py

@@ -28,7 +28,7 @@ def write_imitation_archive(file_name, image_id, repo_tags):
 
     manifest = [
         {
-            'Config': '%s.json' % image_id,
+            'Config': f'{image_id}.json',
             'RepoTags': repo_tags
         }
     ]