Expose dtypes from fused_numerics.pxd for test suite

[lagru]

Description:

We have a nice collection of fused numeric types in fused_numerics.pxd.

• It would be nice to have corresponding lists of these fused types available at runtime from Python. That way we can easily slap the appropriate @pytest.mark.parametrize("dtype", _dtypes_uint) on a test instead of listing each dtype by hand again and again. Could go into _shared.testing in my opinion.
• Check that we used these consistently instead of creating a new local fused type each time. Looking at you _extrema_cy.pyx.

[github-actions]

Hello scikit-image core devs! There hasn't been any activity on this issue for more than 180 days. I have marked it as "dormant" to make it easy to find.
To our contributors, thank you for your contribution and apologies if this issue fell through the cracks! Hopefully this ping will help bring some fresh attention to the issue. If you need help, you can always reach out on our forum
If you think that this issue is no longer relevant, you may close it, or we may do it at some point (either way, it will be done manually).