images.py: deprecation warnings

(Debian) CI reports:

=============================== warnings summary ===============================
tests/test_libmat2.py::TestCleaning::test_all_parametred
  /tmp/autopkgtest-lxc.xekwi_8s/downtmp/build.w7W/src/libmat2/images.py:33: DeprecationWarning: Rsvg.Handle.get_dimensions is deprecated
    dimensions = svg.get_dimensions()

tests/test_libmat2.py::TestCleaning::test_all_parametred
  /tmp/autopkgtest-lxc.xekwi_8s/downtmp/build.w7W/src/libmat2/images.py:38: DeprecationWarning: Rsvg.Handle.render_cairo is deprecated
    svg.render_cairo(context)

-- Docs: https://docs.pytest.org/en/stable/warnings.html