main/py3-docutils: remove dependency on py3-pillow

Pillow is only used by docutils to determine the size of pictures when using
the `scale` attribute on an `image` directive when no `height` and `width`
attributes are specified.

In my opinion, this is a pretty niche use-case, and doesn't warrant a mandatory
dependency. Moreover, docutils also has optional dependencies on Pygments (for
syntax highlighting) and recommonmark (for markdown support), and neither are
marked as dependencies in the Alpine package.

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