DEP: Drop pre-Python 3.6 support

[MartinThoma]

Most importantly, the means Python 2.7 no longer needs to get supported

[codecov]

Codecov Report

Merging #842 (57a662c) into 2.0.0-dev (def7a62) will decrease coverage by 0.27%.
The diff coverage is 93.58%.

@@              Coverage Diff              @@
##           2.0.0-dev     #842      +/-   ##
=============================================
- Coverage      82.47%   82.19%   -0.28%     
=============================================
  Files             16       16              
  Lines           3778     3674     -104     
  Branches         803      780      -23     
=============================================
- Hits            3116     3020      -96     
+ Misses           494      489       -5     
+ Partials         168      165       -3     

Impacted Files	Coverage Δ
PyPDF2/xmp.py	50.00% <0.00%> (-0.28%)	⬇️
PyPDF2/_reader.py	81.14% <66.66%> (-0.10%)	⬇️
PyPDF2/merger.py	66.89% <83.33%> (-0.45%)	⬇️
PyPDF2/generic.py	85.90% <90.90%> (+0.08%)	⬆️
PyPDF2/utils.py	91.91% <96.15%> (-1.70%)	⬇️
PyPDF2/_page.py	79.12% <100.00%> (ø)
PyPDF2/_version.py	100.00% <100.00%> (ø)
PyPDF2/_writer.py	88.74% <100.00%> (ø)
PyPDF2/filters.py	76.23% <100.00%> (-1.29%)	⬇️
PyPDF2/pagerange.py	100.00% <100.00%> (ø)
... and 1 more

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