Remove py2 things from coordinates & topology

[IAlibay]

Towards #2541

Changes made in this Pull Request:

• Removes py2 things (six/future) from coordinates and topology

PR Checklist

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

[codecov]

Codecov Report

Merging #2759 into develop will decrease coverage by 0.04%.
The diff coverage is 67.44%.

@@             Coverage Diff             @@
##           develop    #2759      +/-   ##
===========================================
- Coverage    91.14%   91.09%   -0.05%     
===========================================
  Files          176      177       +1     
  Lines        23711    23692      -19     
  Branches      3119     3122       +3     
===========================================
- Hits         21611    21582      -29     
- Misses        1480     1490      +10     
  Partials       620      620              

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/DLPoly.py	95.20% <ø> (-0.06%)	⬇️
package/MDAnalysis/coordinates/DMS.py	88.40% <ø> (-0.17%)	⬇️
package/MDAnalysis/coordinates/GMS.py	85.61% <ø> (-0.10%)	⬇️
package/MDAnalysis/coordinates/GSD.py	86.66% <0.00%> (-0.57%)	⬇️
package/MDAnalysis/coordinates/INPCRD.py	90.32% <ø> (-0.59%)	⬇️
package/MDAnalysis/coordinates/MMTF.py	100.00% <ø> (ø)
package/MDAnalysis/coordinates/PDBQT.py	84.37% <ø> (-0.17%)	⬇️
package/MDAnalysis/coordinates/ParmEd.py	83.33% <ø> (-0.10%)	⬇️
package/MDAnalysis/coordinates/TXYZ.py	90.00% <0.00%> (-0.33%)	⬇️
package/MDAnalysis/coordinates/__init__.py	100.00% <ø> (ø)
... and 45 more

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