scripts: Temporary workaround for python2

Temporarily patch abc module with ABC helper class until upstream
conventions.py script stops using it.
diff --git a/scripts/lvl_genvk.py b/scripts/lvl_genvk.py
index cfccdda..cd9612e 100644
--- a/scripts/lvl_genvk.py
+++ b/scripts/lvl_genvk.py
@@ -668,6 +668,8 @@
     from layer_dispatch_table_generator import LayerDispatchTableOutputGenerator, LayerDispatchTableGeneratorOptions
     from layer_chassis_generator import LayerChassisOutputGenerator, LayerChassisGeneratorOptions
     from layer_chassis_dispatch_generator import LayerChassisDispatchOutputGenerator, LayerChassisDispatchGeneratorOptions
+    # Temporary workaround for vkconventions python2 compatibility
+    import abc; abc.ABC = abc.ABCMeta('ABC', (object,), {})
     from vkconventions import VulkanConventions
 
     # This splits arguments which are space-separated lists