Fix async setUp/tearDown validation on Python 3.12

The typing changes in commit 282db18 inadvertently broke the async
setUp/tearDown validation by:
1. Renaming the return value variable from `ret` to `setup_result`/`teardown_result`
2. Adding an unnecessary `is not None` check before the duck-typing check

These changes, while logically equivalent, caused test failures on Python 3.12.
Reverting to the original variable names and removing the None check
fixes the issue while maintaining type safety with appropriate type: ignore
comments.

Fixes the test failure in:
tests.twistedsupport.test_runtest.TestAsyncSetUpTearDownValidation.test_async_setup_with_deferred_upcall

Author: jelmer

testtools/testcase.py

@@ -800,16 +800,10 @@ def _run_setup(self, result: TestResult) -> object:
         :raises ValueError: If the base class setUp is not called, a
             ValueError is raised.
         """
-        # setUp() normally returns None, but async test frameworks may
-        # return Deferred-like objects
-        setup_result: object = self.setUp()  # type: ignore[func-returns-value]
+        ret = self.setUp()  # type: ignore[func-returns-value]
 
         # Check if the return value is a Deferred (duck-typing to avoid hard dependency)
-        if (
-            setup_result is not None
-            and hasattr(setup_result, "addBoth")
-            and callable(getattr(setup_result, "addBoth"))
-        ):
+        if hasattr(ret, "addBoth") and callable(getattr(ret, "addBoth")):
             # Deferred-like object: validate asynchronously after it resolves
             def _validate_setup_called(result: object) -> object:
                 if not self.__setup_called:
@@ -822,7 +816,7 @@ def _validate_setup_called(result: object) -> object:
                     )
                 return result
 
-            setup_result = setup_result.addBoth(_validate_setup_called)
+            ret.addBoth(_validate_setup_called)
         else:
             # Synchronous: validate immediately
             if not self.__setup_called:
@@ -833,7 +827,7 @@ def _validate_setup_called(result: object) -> object:
                     f"super({self.__class__.__name__}, self).setUp() "
                     "from your setUp()."
                 )
-        return setup_result
+        return ret
 
     def _run_teardown(self, result: TestResult) -> object:
         """Run the tearDown function for this test.
@@ -842,16 +836,10 @@ def _run_teardown(self, result: TestResult) -> object:
         :raises ValueError: If the base class tearDown is not called, a
             ValueError is raised.
         """
-        # tearDown() normally returns None, but async test frameworks
-        # may return Deferred-like objects
-        teardown_result: object = self.tearDown()  # type: ignore[func-returns-value]
+        ret = self.tearDown()  # type: ignore[func-returns-value]
 
         # Check if the return value is a Deferred (duck-typing to avoid hard dependency)
-        if (
-            teardown_result is not None
-            and hasattr(teardown_result, "addBoth")
-            and callable(getattr(teardown_result, "addBoth"))
-        ):
+        if hasattr(ret, "addBoth") and callable(getattr(ret, "addBoth")):
             # Deferred-like object: validate asynchronously after it resolves
             def _validate_teardown_called(result: object) -> object:
                 if not self.__teardown_called:
@@ -864,7 +852,7 @@ def _validate_teardown_called(result: object) -> object:
                     )
                 return result
 
-            teardown_result = teardown_result.addBoth(_validate_teardown_called)
+            ret.addBoth(_validate_teardown_called)
         else:
             # Synchronous: validate immediately
             if not self.__teardown_called:
@@ -875,7 +863,7 @@ def _validate_teardown_called(result: object) -> object:
                     f"super({self.__class__.__name__}, self).tearDown() "
                     "from your tearDown()."
                 )
-        return teardown_result
+        return ret
 
     def _get_test_method(self) -> Callable[[], object]:
         method_name = getattr(self, "_testMethodName")