[FIX] Sieve Diagram: Using datasets with meta data

[jerneju]

Issue

Widget should intepret meta data which are continuous or discrete in the same way
as features or target.
https://sentry.io/biolab/orange3/issues/231234770/

Description of changes

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3/issues/231234770/

[codecov-io]

Codecov Report

Merging #2098 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2098      +/-   ##
==========================================
- Coverage   71.48%   71.31%   -0.17%     
==========================================
  Files         318      318              
  Lines       54393    54278     -115     
==========================================
- Hits        38881    38709     -172     
- Misses      15512    15569      +57

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[janezd]

The idea here was to avoid unnecessary calls to Discretize. Wouldn't it be better to just replace the above condition with something like

if any(attr.is_continuous for attr in chain(data.domain, data.domain.metas)):

and leave the rest as it is?

[janezd]

This checks just that the widget does not crash. It is difficult to test this widget assuming anything about the widget internals. For the lack of better options, I'd replace self.send_signal("Data", table) with

        from unittest.mock import patch
        from Orange.widgets.visualize.owsieve import Discretize
        with patch("Orange.widgets.visualize.owsieve.Discretize",
                   wraps=Discretize) as disc:
            self.send_signal("Data", table)
            self.assertTrue(disc.called)
        metas = self.widget.discrete_data.domain.metas
        self.assertEqual(len(metas), 2)
        self.assertTrue(all(attr.is_discrete for attr in metas))

Move imports to the beginning of the file, of course.