Reformat imports with isort (#3123)

Besides that:
1. Introduce `setup.py isort` for quick check
2. Introduce GH actions to check imports

tests/test_callback_executor.py

@@ -1,7 +1,9 @@
-import psutil
 import unittest
+
 from unittest.mock import Mock, patch
 
+import psutil
+
 from patroni.postgresql.callback_executor import CallbackExecutor