test_owcurvefit: fix testing failure on conda

[markotoplak]

Issue

I saw the follwing with conda scipy from conda (first noticed in biolab/orange3-installers#55 (comment)):

======================================================================
FAIL: test_expression (Orange.widgets.model.tests.test_owcurvefit.TestOWCurveFit.test_expression)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\test-install\Lib\site-packages\Orange\widgets\model\tests\test_owcurvefit.py", line 373, in test_expression
    self.assertIsNotNone(model)
AssertionError: unexpectedly None

Description of changes

test_expression looped through all possible functions, and some of them produce objective function values with NaNs.

Different implementations of optimization that scipy uses get different results: one stops optimization immediately with some results, the other stops when maximum number of iterations (and errors with fitting_failed). The optimization implementation can be different even with the same scipy version (in my case, between pypi and conda-forge package). Thus, we skip these.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.39%. Comparing base (871b406) to head (c84d9c4).
Report is 124 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6928   +/-   ##
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  Coverage   88.39%   88.39%           
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  Files         329      329           
  Lines       72480    72544   +64     
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+ Hits        64069    64128   +59     
- Misses       8411     8416    +5