Reformat imports with isort (#3123)

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

.github/workflows/tests.yaml

@@ -211,3 +211,20 @@ jobs:
 
     - name: Generate documentation
       run: tox -m docs
+
+  isort:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python 3.12
+      uses: actions/setup-python@v5
+      with:
+        python-version: 3.12
+        cache: pip
+
+    - name: isort
+      uses: isort/isort-action@master
+      with:
+        requirementsFiles: "requirements.txt requirements.dev.txt requirements.docs.txt"
+        sort-paths: "patroni tests features setup.py"