FTBFS against python 3.11.7 due to use of module "meant for internal use by Python only"

Attempting to build ubiquity on noble fails like so:

+======================================================================
+ERROR: test_ubi_partman (unittest.loader._FailedTest.test_ubi_partman)
+----------------------------------------------------------------------
+ImportError: Failed to import test module: test_ubi_partman
+Traceback (most recent call last):
+ File "/usr/lib/python3.11/unittest/loader.py", line 162, in loadTestsFromName
+ module = __import__(module_name)
+ ^^^^^^^^^^^^^^^^^^^^^^^
+ File "/tmp/autopkgtest.NpvRQz/build.mFW/src/tests/test_ubi_partman.py", line 5, in <module>
+ from test.support import run_unittest
+ImportError: cannot import name 'run_unittest' from 'test.support' (/usr/lib/python3.11/test/support/__init__.py)

The test module has the following note:

Note
The test package is meant for internal use by Python only. It is documented for the benefit of the core developers of Python. Any use of this package outside of Python’s standard library is discouraged as code mentioned here can change or be removed without notice between releases of Python.

run_unittest is no longer offered by test.support, so this is a Ubiquity bug for using this internal-use only module.

For the purposes of Ubuntu Noble, this change appears as of python 3.11.7.