I am uploading a NMU to DELAYED/10 in order to fix this.
The debdiff is attached.
diff -Nru mrb-0.3+nmu2/debian/changelog mrb-0.3+nmu3/debian/changelog
--- mrb-0.3+nmu2/debian/changelog 2022-04-21 15:33:07.000000000 +0200
+++ mrb-0.3+nmu3/debian/changelog 2023-09-29 14:30:06.000000000 +0200
@@ -1,3 +1,11 @@
+mrb (0.3+nmu3) unstable; urgency=medium
+
+ * Non-maintainer upload.
+ * Convert to source format 3.0.
+ Closes: #1043215
+
+ -- Bastian Germann <b...@debian.org> Fri, 29 Sep 2023 12:30:06 +0000
+
mrb (0.3+nmu2) unstable; urgency=medium
* Non-maintainer upload.
diff -Nru mrb-0.3+nmu2/debian/source/format mrb-0.3+nmu3/debian/source/format
--- mrb-0.3+nmu2/debian/source/format 1970-01-01 01:00:00.000000000 +0100
+++ mrb-0.3+nmu3/debian/source/format 2023-09-29 14:30:06.000000000 +0200
@@ -0,0 +1 @@
+3.0 (native)
