Add 34 prefix value (ar localflavor ARCUITField).

[ramiro]

There are now CUIT assigned that use a 34 prefix. This PR adds support for it.

All Changes

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

• [x] Add an entry for your name in the docs/authors.rst file if it's not already there.

• [x] Adjust your imports to a standard form by running this command:

  isort --recursive --line-width 120 localflavor tests

[ramiro]

Thank you for your reviews! I think I've taken care of them.

[codecov-io]

Codecov Report

Merging #342 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #342   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files         153      153           
  Lines        3878     3878           
  Branches      517      517           
=======================================
  Hits         3716     3716           
  Misses         98       98           
  Partials       64       64

Impacted Files	Coverage Δ
localflavor/ar/forms.py	92.85% <100%> (ø)	:arrow_up:

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