Unbreak parser source context dumps in server tracebacks
This is a regression from where `MarkupExceptionContext` was
erroneously left without the `MarkupCapableMixin` in its bases.
@@ -38,7 +38,10 @@ class MarkupCapableMixin:
serializer.serializer.register(cls)(cls.as_markup)
-class MarkupExceptionContext(exceptions.ExceptionContext):
+class MarkupExceptionContext(
+ exceptions.ExceptionContext,
+ MarkupCapableMixin,
+):
@abc.abstractclassmethod
def as_markup(cls, *, ctx):
