Switch ARCBUField to use python-stdnum

[arthurdejong]

This switches the (incorrect) implementation in ARCBUField to use the validation logic of python-stdnum.

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• [x] Add an entry to the docs/changelog.rst describing the change.

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[codecov-io]

Codecov Report

Merging #391 into master will decrease coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   95.87%   95.85%   -0.02%     
==========================================
  Files         161      161              
  Lines        3997     3983      -14     
  Branches      530      528       -2     
==========================================
- Hits         3832     3818      -14     
  Misses         99       99              
  Partials       66       66

Impacted Files	Coverage Δ
localflavor/ar/forms.py	91.46% <100%> (-1.25%)	:arrow_down:

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[arthurdejong]

Just one small request: Could you add a .. versionchanged:: 3.0 to the docstring? Thanks.

Done.

[benkonrath]

Thanks!