diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e951649f41a7f17c0433e0f30803de8b7805925b..a95066391320a6e9591228784671faa5c5056cc8 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -5,11 +5,11 @@
     - docker
     - linux
 
-flake8:
+black:
   <<: *test_job
   image: registry.gitlab.isc.org/isc-projects/images/bind9:debian-bullseye-amd64
   script:
-    - find . -name "*.py" -execdir flake8 {} +
+    - black $(git ls-files '*.py')
 
 pylint:
   <<: *test_job
diff --git a/windows-docker/executor.py b/windows-docker/executor.py
index ff6a5d1bff8493a3ca7abee3cec63470f571b928..67093230a0b70b4132d59afeec87afe3223a103e 100644
--- a/windows-docker/executor.py
+++ b/windows-docker/executor.py
@@ -17,7 +17,7 @@ import subprocess
 import sys
 
 
-DOCKER_PATH = 'C:\\Program Files\\Docker\\docker.exe'
+DOCKER_PATH = "C:\\Program Files\\Docker\\docker.exe"
 
 
 def fatal_error(message):
@@ -43,9 +43,9 @@ def do_config(*_):
     Perform the Config stage of a GitLab CI job.
     """
     config = {
-        'builds_dir': 'C:\\builds',
-        'cache_dir': 'C:\\cache',
-        'builds_dir_is_shared': False,
+        "builds_dir": "C:\\builds",
+        "cache_dir": "C:\\cache",
+        "builds_dir_is_shared": False,
     }
     print(json.dumps(config))
 
@@ -54,12 +54,17 @@ def do_prepare(container_name, _):
     """
     Perform the Prepare stage of a GitLab CI job.
     """
-    docker_command(['run',
-                    '--name', container_name,
-                    '--interactive',
-                    '--detach',
-                    '--rm',
-                    os.environ['CUSTOM_ENV_CI_JOB_IMAGE']])
+    docker_command(
+        [
+            "run",
+            "--name",
+            container_name,
+            "--interactive",
+            "--detach",
+            "--rm",
+            os.environ["CUSTOM_ENV_CI_JOB_IMAGE"],
+        ]
+    )
 
 
 def do_run(container_name, args):
@@ -67,17 +72,17 @@ def do_run(container_name, args):
     Perform the Run stage of a GitLab CI job.
     """
     valid_stages = [
-        'prepare_script',
-        'get_sources',
-        'restore_cache',
-        'download_artifacts',
-        'build_script',
-        'step_script',
-        'after_script',
-        'archive_cache',
-        'upload_artifacts_on_success',
-        'upload_artifacts_on_failure',
-        'cleanup_file_variables',
+        "prepare_script",
+        "get_sources",
+        "restore_cache",
+        "download_artifacts",
+        "build_script",
+        "step_script",
+        "after_script",
+        "archive_cache",
+        "upload_artifacts_on_success",
+        "upload_artifacts_on_failure",
+        "cleanup_file_variables",
     ]
 
     script_src = args[0]
@@ -86,17 +91,17 @@ def do_run(container_name, args):
     if stage not in valid_stages:
         fatal_error(f'Unsupported run stage "{stage}"')
 
-    script_dst_path = f'C:\\{stage}.ps1'
-    script_dst = f'{container_name}:{script_dst_path}'
-    docker_command(['cp', script_src, script_dst])
-    docker_command(['exec', container_name, 'powershell.exe', script_dst_path])
+    script_dst_path = f"C:\\{stage}.ps1"
+    script_dst = f"{container_name}:{script_dst_path}"
+    docker_command(["cp", script_src, script_dst])
+    docker_command(["exec", container_name, "powershell.exe", script_dst_path])
 
 
 def do_cleanup(container_name, _):
     """
     Perform the Cleanup stage of a GitLab CI job.
     """
-    docker_command(['stop', container_name])
+    docker_command(["stop", container_name])
 
 
 def main():
@@ -104,21 +109,21 @@ def main():
     Parse arguments and perform the requested stage of a GitLab CI job.
     """
     actions = {
-        'config': do_config,
-        'prepare': do_prepare,
-        'run': do_run,
-        'cleanup': do_cleanup,
+        "config": do_config,
+        "prepare": do_prepare,
+        "run": do_run,
+        "cleanup": do_cleanup,
     }
-    valid_actions = '|'.join(actions.keys())
+    valid_actions = "|".join(actions.keys())
 
     try:
         action = actions[sys.argv[1]]
     except (IndexError, KeyError):
-        fatal_error(f'Usage: {sys.argv[0]} {valid_actions} [args...]')
+        fatal_error(f"Usage: {sys.argv[0]} {valid_actions} [args...]")
 
-    container_name = 'gitlab-runner-' + os.environ['CUSTOM_ENV_CI_JOB_ID']
+    container_name = "gitlab-runner-" + os.environ["CUSTOM_ENV_CI_JOB_ID"]
     action(container_name, sys.argv[2:])
 
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     main()