Hi!

While rebuilding the python related packages against the Python 3.15rc2
version we found that pytest-mypy-plugins fails to build from source:
https://debusine.debian.net/debian/r-python-python3.15/artifact/4855925/

In the log, we can see that the calls to subprocess fail, but because
the output of the call is sent to /dev/null, we don't know what caused
the failure. However, given that this seems to be looking at error
messages, it's likely that the new Python 3.15 error messages don't
match the expected ones.

=================================== FAILURES
===================================

pytest_mypy_plugins/tests/test_site_packages.py:117: AssertionError

pytest_mypy_plugins/tests/test_site_packages.py:156: AssertionError

Setting the severity to important for now. Once Python 3.15 is released
and later added to python3-defaults, this bug will become release
critical.