configfile: Improve support for python3.12

It seems python3.12 has removed support for readfp() - use read_file()
instead.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/configfile.py

@@ -217,6 +217,9 @@ class PrinterConfig:
         data = '\n'.join(buffer)
         del buffer[:]
         sbuffer = io.StringIO(data)
+        if sys.version_info.major >= 3:
+            fileconfig.read_file(sbuffer, filename)
+        else:
             fileconfig.readfp(sbuffer, filename)
     def _resolve_include(self, source_filename, include_spec, fileconfig,
                          visited):