Remove python2 compatibility decorators

[jeremy-engel]

Adds support for django 3.0

See https://docs.djangoproject.com/en/3.0/releases/3.0/#removed-private-python-2-compatibility-apis

The README indicates that python2 is not supported in the current
version, so there shouldn't by any issues removing these.

[coveralls]

Coverage increased (+0.005%) to 84.629% when pulling 294656e on jeremy-engel:django-3.0-compat into 0c10b4c on jedie:master.

[codecov]

Codecov Report

Merging #113 into master will decrease coverage by 0.56%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #113      +/-   ##
=========================================
- Coverage   78.47%   77.9%   -0.57%     
=========================================
  Files           4       4              
  Lines         432     439       +7     
  Branches      102     103       +1     
=========================================
+ Hits          339     342       +3     
- Misses         69      73       +4     
  Partials       24      24

Impacted Files	Coverage Δ
reversion_compare/compare.py	84.5% <50%> (-1.42%)	⬇️

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[devlounge]

When could this be merged ?

[jedie]

Thanks for contributing

[jedie]

it's included in the v0.9 release: https://pypi.org/project/django-reversion-compare/0.9.0/