Ignore security issue with `mkdocs-material`

This requires handling upstream (see linked issue), trying to bump this
dependency errored with:

    Because mkdocs-material (9.5.32) depends on mkdocs (>=1.6,<2.0)
     and portray (1.8.0) depends on mkdocs (>=1.3.0,<1.4.0), mkdocs-material (9.5.32) is incompatible with portray (1.8.0).
    And because no versions of portray match >1.8.0, mkdocs-material (9.5.32) is incompatible with portray (>=1.8.0).
    So, because isort depends on both portray (>=1.8.0) and mkdocs-material (9.5.32), version solving failed.