Simplify props.py

[RJPercival]

It had code for handling the differences between old-style and new-style classes in Python 2, but support for Python 2 was dropped long ago.

This could be simplified further, e.g. by skipping any attributes that start with an underscore, but that would be a behavioural change, whereas this PR is a pure refactoring.

[erikwrede]

Thanks for the PR @RJPercival 🙂 Could you please run the pre-commit hooks before commiting, so that the linter runs? Feel free to tag me after the PR is updated, will merge then. 😊

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (d77d0b0) 96.01% compared to head (6d609db) 96.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1511      +/-   ##
==========================================
- Coverage   96.01%   96.00%   -0.01%     
==========================================
  Files          51       51              
  Lines        1755     1753       -2     
==========================================
- Hits         1685     1683       -2     
  Misses         70       70              

Impacted Files	Coverage Δ
graphene/utils/props.py	100.00% <100.00%> (ø)

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