travis: update versions for test

[PrimozGodec]

Issue

• Add-on not tested on Python 3.7.
• Orange 3.16 is very old and does not contain all required components.

Description of changes

• Testing on Python 3.7
• Oldest supported version of Orange changed to 3.20.

Includes

• [ ] Code changes
• [X] Tests
• [ ] Documentation

[codecov-io]

Codecov Report

Merging #72 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #72   +/-   ##
======================================
  Coverage    85.1%   85.1%           
======================================
  Files          17      17           
  Lines        2276    2276           
  Branches      279     279           
======================================
  Hits         1937    1937           
  Misses        305     305           
  Partials       34      34

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