Use backports.csv only for py2

[meeuw]

Description

Fixed by Terje Røsten
https://bugzilla.redhat.com/attachment.cgi?id=1314388&action=diff

The rationale behind not installing backports.csv for Python 3 is that it isn't available for Fedora.

Checklist

• I've added this contribution to the CHANGELOG.
• I've added my name to the AUTHORS file (or it's already there).

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 1.08%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #24      +/-   ##
========================================
+ Coverage   98.91%   100%   +1.08%     
========================================
  Files          10     10              
  Lines         276    276              
========================================
+ Hits          273    276       +3     
+ Misses          3      0       -3

Impacted Files	Coverage Δ
cli_helpers/compat.py	100% <0%> (+11.53%)	⬆️

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[tsroten]

@meeuw We'll need to take out the universal = 1 line from setup.cfg. Then, during a release, I'll build separate Python 2 and Python 3 wheels.

[meeuw]

allright, rebased and removed [bdist_wheel] (I hope that's what you mean)