CSV File Import: pylint

[ales-erjavec]

Issue

Pylint the files added in gh-3876

Description of changes

Also fix other issues:

• CSV File Import needs pandas as a requirement
• One branch in TablePreviewModel.data had a missing return statement

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3888 into master will increase coverage by <.01%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master    #3888      +/-   ##
==========================================
+ Coverage   84.28%   84.28%   +<.01%     
==========================================
  Files         384      384              
  Lines       72736    72739       +3     
==========================================
+ Hits        61307    61310       +3     
  Misses      11429    11429

[janezd]

Thanks! This file really needed some cleaning up.