Bug #296: python 3.12 compability

Since python 3.2, readfp needs to be replaced by read_file.
Python 3.12 dropped the readfp function.

Patch provided as PR by me, as the original reporter failed to do
so since 4-4-2024 and my systems break due to this issue.

ifupdown2/ifupdown/main.py

@@ -138,7 +138,7 @@ def read_config(self):
         configStr = '[ifupdown2]\n' + config
         configFP = io.StringIO(configStr)
         parser = configparser.RawConfigParser()
-        parser.readfp(configFP)
+        parser.read_file(configFP)
         configmap_g = dict(parser.items('ifupdown2'))
 
         # Preprocess config map