Add support for Python 3.12

Python 3.12 configparser removed SafeConfigParser.
python/cpython#89336 (comment)

leading to this issue

```
Traceback (most recent call last):
  File "/home/user/.cache/pypoetry/virtualenvs/chapter-marker-bSlZRqHo-py3.12/bin/pflake8", line 5, in <module>
    from pflake8.__main__ import main
  File "/home/user/.cache/pypoetry/virtualenvs/chapter-marker-bSlZRqHo-py3.12/lib/python3.12/site-packages/pflake8/__init__.py", line 54, in <module>
    class DivertingSafeConfigParser(ConfigParserTomlMixin, configparser.SafeConfigParser):
                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'configparser' has no attribute 'SafeConfigParser'. Did you mean: 'RawConfigParser'
```

pflake8/__init__.py

@@ -52,13 +52,20 @@ class DivertingConfigParser(ConfigParserTomlMixin, configparser.ConfigParser):
     pass
 
 
-class DivertingSafeConfigParser(ConfigParserTomlMixin, configparser.SafeConfigParser):
-    pass
+try:
+
+    class DivertingSafeConfigParser(
+        ConfigParserTomlMixin, configparser.SafeConfigParser
+    ):
+        pass
+
+    configparser.SafeConfigParser = DivertingSafeConfigParser
+except AttributeError:
+    pass  # does not exist on Python 3.12 (https://github.com/python/cpython/issues/89336#issuecomment-1094366625)
 
 
 configparser.RawConfigParser = DivertingRawConfigParser
 configparser.ConfigParser = DivertingConfigParser
-configparser.SafeConfigParser = DivertingSafeConfigParser
 
 
 class FixFilenames(ast.NodeTransformer):