Remove py2 things from analysis

[IAlibay]

Fixes #2541

Changes made in this Pull Request:

• Removes six & future imports from analysis

Note:

• Omitted hbond_analysis & hole as set for removal

Question:

• test_wbridge doesn't have any of the file header blurb, do we want it add it in?

PR Checklist

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

[codecov]

Codecov Report

Merging #2753 into develop will decrease coverage by 0.01%.
The diff coverage is 73.80%.

@@             Coverage Diff             @@
##           develop    #2753      +/-   ##
===========================================
- Coverage    91.17%   91.16%   -0.02%     
===========================================
  Files          176      176              
  Lines        23798    23734      -64     
  Branches      3134     3135       +1     
===========================================
- Hits         21699    21636      -63     
+ Misses        1476     1475       -1     
  Partials       623      623              

Impacted Files	Coverage Δ
package/MDAnalysis/analysis/contacts.py	100.00% <ø> (ø)
package/MDAnalysis/analysis/data/filenames.py	100.00% <ø> (ø)
package/MDAnalysis/analysis/diffusionmap.py	93.10% <ø> (-0.23%)	⬇️
package/MDAnalysis/analysis/dihedrals.py	96.66% <ø> (-0.06%)	⬇️
package/MDAnalysis/analysis/distances.py	97.50% <ø> (-0.07%)	⬇️
package/MDAnalysis/analysis/encore/__init__.py	100.00% <ø> (ø)
package/MDAnalysis/analysis/encore/bootstrap.py	100.00% <ø> (ø)
...sis/analysis/encore/clustering/ClusteringMethod.py	96.87% <ø> (-0.05%)	⬇️
.../MDAnalysis/analysis/encore/clustering/__init__.py	100.00% <ø> (ø)
...e/MDAnalysis/analysis/encore/clustering/cluster.py	96.15% <ø> (-0.08%)	⬇️
... and 31 more

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[IAlibay]

As per #2752 (comment), I've reverted to raising from None.