Test with Python 3.11

[sir-sigurd]

No description provided.

[kleisauke]

FWIW, this warning:

=============================== warnings summary ===============================
tests/test_progress.py::TestProgress::test_progress_fields
  /home/runner/work/pyvips/pyvips/.tox/py/lib/python3.11/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning: Exception ignored from cffi callback <function _marshal_image_progress at 0x7f6e4797e3e0>: None
  
  Traceback (most recent call last):
    File "/home/runner/work/pyvips/pyvips/pyvips/gobject.py", line 32, in _marshal_image_progress
      callback(image, progress)
    File "/home/runner/work/pyvips/pyvips/tests/test_progress.py", line 45, in posteval_cb
      assert progress.percent == 100
  AssertionError: assert 99 == 100
   +  where 99 = <cdata 'VipsProgress *' 0x558b48ee56e0>.percent
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=================== [44](https://github.com/libvips/pyvips/actions/runs/4772461785/jobs/8484994109#step:7:45) passed, 9 skipped, 1 warning in 1.77s ===================

Can safely be ignored as it was fixed with commit libvips/libvips@111eef0 (released in libvips 8.13.0).

[jcupitt]

Sorry for the delay, thank you for doing this work, @sir-sigurd !