Fix linting and testing errors (#272)

- Fixes Black formatting and import order in `django-stubs/db/transaction.pyi`.
- ~~Fixes expected Pyright error messages in `tests/pyright/test_arrays.py` and `tests/pyright/test_fields.py`.~~ Edit: With Python pinned to 3.8.18, these message changes are no longer necessary.

These changes allow `./s/lint` to run successfully on my machine.

.github/workflows/test.yml

@@ -19,7 +19,6 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: "3.8"
           cache: "poetry"
 
       - name: Use Node.js

.python-version

@@ -1 +1 @@
-3.8.19
+3.8.18

django-stubs/db/transaction.pyi

@@ -1,8 +1,8 @@
 from collections.abc import Callable, Iterator
 from contextlib import contextmanager
+from types import TracebackType
 from typing import Any, TypeVar, overload
 
-from types import TracebackType
 from django.db import ProgrammingError
 
 class TransactionManagementError(ProgrammingError): ...
@@ -40,8 +40,9 @@ class Atomic:
         self,
         exc_type: type[BaseException] | None,
         exc_value: BaseException | None,
-        traceback: TracebackType | None, /
+        traceback: TracebackType | None,
     ) -> None: ...
+
 # Bare decorator
 @overload
 def atomic(using: _C) -> _C: ...