skimage.io use is deprecated

[nkeim]

Starting with skimage 0.25, the plugin argument to e.g. imread() is deprecated and will be removed in 0.27. Using the imageio package directly is the favored replacement. A simple read operation in pims will give an error like this:

pims/image_sequence.py:85: FutureWarning: The plugin infrastructure in `skimage.io` and the parameter `plugin` are deprecated since version 0.25 and will be removed in 0.27 (or later). To avoid this warning, please do not use the parameter `plugin`. Instead, use `imageio` or other I/O packages directly. See also `imread`.
  return imread(filename, **kwargs)

A discussion of the change is here: https://discuss.scientific-python.org/t/long-term-plan-for-skimage-io-plugin-infrastructure/1079

This should be a straightforward change for pims, if we can get to it soon. Off the top of my head, it means

• Checking the reader resolution order to make sure imageio is preferred over skimage
• Adding imageio as a package requirement
• Trying to drop skimage as a package requirement

In the future, we would drop support for skimage readers altogether (rather than check the version).