Handle breaking api change in Python 3.12.1

In Python 3.12.1 python/cpython#106588 was backported which changed the
execution behavior of the unittest runner. After python/cpython#106588
startTest() is no longer being called if a test is skipped. This causes
knock-on effects in testtools because the test result subclasses were
assuming that startTest() was always called when stopTest() was called.
To handle this change in behavior when running with Python 3.12.1 this
commit adds a check to only deal with tags if they exist (when
startTest() is run).

testtools/testresult/real.py

@@ -227,7 +227,9 @@ def startTest(self, test):
         self._tags = TagContext(self._tags)
 
     def stopTest(self, test):
-        self._tags = self._tags.parent
+        # NOTE: In Python 3.12.1 skipped tests may not call startTest()
+        if self._tags is not None:
+            self._tags = self._tags.parent
         super().stopTest(test)
 
     @property
@@ -1608,7 +1610,9 @@ def stop(self):
         self.shouldStop = True
 
     def stopTest(self, test):
-        self._tags = self._tags.parent
+        # NOTE: In Python 3.12.1 skipped tests may not call startTest()
+        if self._tags is not None:
+            self._tags = self._tags.parent
         return self.decorated.stopTest(test)
 
     def stopTestRun(self):
@@ -1670,7 +1674,9 @@ def startTest(self, test):
         self._tags = TagContext(self._tags)
 
     def stopTest(self, test):
-        self._tags = self._tags.parent
+        # NOTE: In Python 3.12.1 skipped tests may not call startTest()
+        if self._tags is not None:
+            self._tags = self._tags.parent
 
     def addError(self, test, err=None, details=None):
         self._check_args(err, details)