add week to the dutch/nl locale

[robinvd]

Pull Request Checklist

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

Description of Changes

Add weeks to the dutch locale

[codecov-io]

Codecov Report

Merging #741 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #741   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1678   1678           
  Branches      283    283           
=====================================
  Hits         1678   1678

Impacted Files	Coverage Δ
arrow/locales.py	100% <ø> (ø)	⬆️

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[jadchaar]

LGTM