gst-validate: Fix compatibility with Python 3.12

I recently upgraded my development system to Fedora 39 which has python 3.12. Now, when running gst-validate-launcher using something like:

meson devenv -w "$PWD" -C ./builddir gst-validate-launcher --no-display check.gst-plugins-base

I get the error:

AttributeError: 'RawConfigParser' object has no attribute 'readfp'. Did you mean: 'read'?

config.readfp() was removed in python 3.12 and it looks like config.read_file() does the same thing and has been available since Python 3.2 so it should be a good replacement.