Please find attached the debdiff fixing this bug.
diff -Nru twisted-24.3.0/debian/changelog twisted-24.3.0/debian/changelog
--- twisted-24.3.0/debian/changelog 2024-03-04 15:57:17.000000000 +0100
+++ twisted-24.3.0/debian/changelog 2024-06-21 11:20:23.000000000 +0200
@@ -1,3 +1,10 @@
+twisted (24.3.0-2) unstable; urgency=medium
+
+ * Patch: fix test_flatten in autopkgtest. (Closes: #1073996)
+ This patch can probably be dropped after next upstream release.
+
+ -- Florent 'Skia' Jacquet <florent.jacq...@canonical.com> Fri, 21 Jun 2024
11:20:23 +0200
+
twisted (24.3.0-1) unstable; urgency=medium
* New upstream release.
diff -Nru twisted-24.3.0/debian/patches/series
twisted-24.3.0/debian/patches/series
--- twisted-24.3.0/debian/patches/series 2024-03-04 15:57:17.000000000
+0100
+++ twisted-24.3.0/debian/patches/series 2024-06-21 11:20:23.000000000
+0200
@@ -15,3 +15,4 @@
debian-hacks/Drop-dependency-on-typing_extensions.patch
debian-hacks/reproducible-docs.patch
tests/skip-test_sendFileDescriptorTriggersPauseProducing.patch
+tests/Tests-Fix-test_flatten.patch
diff -Nru twisted-24.3.0/debian/patches/tests/Tests-Fix-test_flatten.patch
twisted-24.3.0/debian/patches/tests/Tests-Fix-test_flatten.patch
--- twisted-24.3.0/debian/patches/tests/Tests-Fix-test_flatten.patch
1970-01-01 01:00:00.000000000 +0100
+++ twisted-24.3.0/debian/patches/tests/Tests-Fix-test_flatten.patch
2024-06-21 11:20:23.000000000 +0200
@@ -0,0 +1,34 @@
+From: Florent 'Skia' Jacquet <florent.jacq...@canonical.com>
+Date: Fri, 21 Jun 2024 12:06:48 +0200
+Subject: Test: Fix twisted.web.test.test_flatten
+
+Following some removal from Python itself, the following started to appear in
autopkgtest:
+
+2993s --- <exception caught here> ---
+2993s File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line
1999, in _inlineCallbacks
+2993s result = context.run(
+2993s File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line
519, in throwExceptionIntoGenerator
+2993s return g.throw(self.value.with_traceback(self.tb))
+2993s File "/usr/lib/python3/dist-packages/twisted/web/_flatten.py", line
435, in _flattenTree
+2993s roots.append(frame.f_locals["root"])
+2993s builtins.KeyError: 'root'
+
+This is fixed upstream in https://github.com/twisted/twisted/pull/12213
+This patch is a cherry-pick of the needed changes, and can probably be dropped
next release.
+--- a/src/twisted/web/_flatten.py
++++ b/src/twisted/web/_flatten.py
+@@ -428,11 +427,11 @@
+ except StopIteration:
+ stack.pop()
+ except Exception as e:
+- stack.pop()
+ roots = []
+ for generator in stack:
+- roots.append(generator.gi_frame.f_locals["root"])
+- roots.append(frame.f_locals["root"])
++ if generator.gi_frame is not None:
++ roots.append(generator.gi_frame.f_locals["root"])
++ stack.pop()
+ raise FlattenerError(e, roots, extract_tb(exc_info()[2]))
+ else:
+ stack.append(element)
