The prefix for GMarkupParseFlags enumeration members is G_MARKUP; this
means that G_MARKUP_PARSE_FLAGS_NONE gets split into
GLib.MarkupParseFlags.PARSE_FLAGS_NONE by the introspection scanner.

The `/*< nick=none >*/` trigraph attribute is a glib-mkenum thing, and
does not affect the introspection scanner; it would also only affect the
GEnumValue nickname, which is not used by language bindings to resolve
the name of the enumeration member. Plus, GMarkupParseFlags does not
have a corresponding GType anyway.