`tests/helpers.py` is missing in 3.2.0 PyPI sdist

[mgorny]

When running the test suite from PyPI sdist for 3.2.0, I'm getting errors like the following:

_______________________________________________ ERROR collecting tests/test_ulinalg.py ________________________________________________
ImportError while importing test module '/tmp/portage/dev-python/uncertainties-3.2.0/work/uncertainties-3.2.0/tests/test_ulinalg.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.10/site-packages/_pytest/python.py:492: in importtestmodule
    mod = import_path(
/usr/lib/python3.10/site-packages/_pytest/pathlib.py:591: in import_path
    importlib.import_module(module_name)
/usr/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1050: in _gcd_import
    ???
<frozen importlib._bootstrap>:1027: in _find_and_load
    ???
<frozen importlib._bootstrap>:1006: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:688: in _load_unlocked
    ???
/usr/lib/python3.10/site-packages/_pytest/assertion/rewrite.py:178: in exec_module
    exec(co, module.__dict__)
tests/test_ulinalg.py:11: in <module>
    from helpers import uarrays_close
E   ModuleNotFoundError: No module named 'helpers'

Indeed, tests/helpers.py is not included in the sdist.