DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats

[kenoss]

pytest outputs the following warning:

============================================================== warnings summary ==============================================================
tests/intergation_test/test_gstreamer_output.py: 52 warnings
  /home/keno/.cache/pypoetry/virtualenvs/actfw-gstreamer-geNW1jjz-py3.9/lib/python3.9/site-packages/PIL/Image.py:812: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
    s = d.decode(data)

tests/intergation_test/test_gstreamer_output.py::test_videotestsrc
tests/intergation_test/test_gstreamer_output.py::test_videotestsrc
tests/intergation_test/test_gstreamer_output.py::test_videotestsrc
tests/intergation_test/test_gstreamer_output.py::test_videotestsrc
  /home/keno/.cache/pypoetry/virtualenvs/actfw-gstreamer-geNW1jjz-py3.9/lib/python3.9/site-packages/PIL/ImageFile.py:241: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
    n, err_code = decoder.decode(b)

-- Docs: https://docs.pytest.org/en/stable/warnings.html
====================================================== 12 passed, 56 warnings in 9.22s =======================================================

This comes from Pillow < 6.0.0.
This does not affect our use cases until an update of apt packages comes.

Action: Wait.