Remove py2 things from core

[IAlibay]

Towards #2541

Changes made in this Pull Request:

• Removes py2 things from core

PR Checklist

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

[codecov]

Codecov Report

Merging #2756 into develop will decrease coverage by 0.00%.
The diff coverage is 79.41%.

@@             Coverage Diff             @@
##           develop    #2756      +/-   ##
===========================================
- Coverage    91.17%   91.17%   -0.01%     
===========================================
  Files          176      176              
  Lines        23798    23806       +8     
  Branches      3134     3134              
===========================================
+ Hits         21699    21705       +6     
- Misses        1476     1480       +4     
+ Partials       623      621       -2     

Impacted Files	Coverage Δ
package/MDAnalysis/core/__init__.py	100.00% <ø> (ø)
package/MDAnalysis/core/topology.py	100.00% <ø> (ø)
package/MDAnalysis/core/topologyattrs.py	95.29% <33.33%> (-0.12%)	⬇️
package/MDAnalysis/core/groups.py	97.80% <68.75%> (-0.35%)	⬇️
package/MDAnalysis/core/_get_readers.py	96.05% <77.77%> (+0.05%)	⬆️
package/MDAnalysis/core/selection.py	99.48% <100.00%> (+<0.01%)	⬆️
package/MDAnalysis/core/topologyobjects.py	97.50% <100.00%> (-0.01%)	⬇️
package/MDAnalysis/core/universe.py	95.86% <100.00%> (-0.02%)	⬇️
core/__init__.py	100.00% <0.00%> (ø)
package/MDAnalysis/lib/transformations.py	78.62% <0.00%> (+0.43%)	⬆️

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