Pylint checks for Python 3 only

[lilyminium]

Fixes #2744

Changes made in this Pull Request:

• get the linter check to error on Python 3 things only

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[IAlibay]

seems to be working, lgtm

[codecov]

Codecov Report

Merging #2750 into develop will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2750      +/-   ##
===========================================
- Coverage    91.31%   91.30%   -0.01%     
===========================================
  Files          176      176              
  Lines        24018    24016       -2     
  Branches      3160     3160              
===========================================
- Hits         21931    21929       -2     
  Misses        1459     1459              
  Partials       628      628              

Impacted Files	Coverage Δ
package/MDAnalysis/analysis/align.py	95.32% <100.00%> (-0.04%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3da643f...4736497. Read the comment docs.

[lilyminium]

We can either merge all __future__ cleanups into this PR or merge this into develop first and have separate ones for clean-up. I haven't checked if the old linter looks for six etc.

[orbeckst]

lgtm, thanks

[orbeckst]

I know that I previously said to remove everything in one go together with the check. But looking at the PR, it seems more conducive to moving things along if we merged it right away and then people can chip away at ripping out Py 2 when they feel they need some light distraction that still passes as doing something productive.