[FIX] owlouvainclustering: Make the task completion handler a single slot

[ales-erjavec]

Issue

The
Orange.widgets.unsupervised.tests.test_owlouvain.TestOWLouvain.test_clusters_ordered_by_size test errors randomly with:

======================================================================
ERROR: test_clusters_ordered_by_size (Orange.widgets.unsupervised.tests.test_owlouvain.TestOWLouvain)
Cluster names should be sorted based on the number of instances.
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Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/unsupervised/tests/test_owlouvain.py", line 47, in test_clusters_ordered_by_size
    clustering = output.get_column_view('Cluster')[0].astype(int)
AttributeError: 'NoneType' object has no attribute 'get_column_view'

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Description of changes

Make the task completion handler a single slot.

The handlers are invoked with the Qt.QueuedConnection semantics and the first (_processing_complete) advertised to the world that the processing has completed, but the actual send would be done later at an unspecified time. This tripped the TestOWLouvain.test_clusters_ordered_by_size test.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3182 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3182      +/-   ##
==========================================
+ Coverage   82.63%   82.63%   +<.01%     
==========================================
  Files         340      340              
  Lines       58788    58790       +2     
==========================================
+ Hits        48577    48579       +2     
  Misses      10211    10211