Added translation for weeks in Finnish locale

[vreima]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

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

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Description of Changes

Added two lines to Finnish locale for the humanization of the 'week' granularity. arrow.utcnow().shift(weeks=2).humanize(locale='fi') used to raise ValueError.

[jadchaar]

LGTM

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (de0aea9) 100.00% compared to head (d2fee43) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1157   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2326      2326           
  Branches       400       400           
=========================================
  Hits          2326      2326           

Files	Coverage Δ
arrow/locales.py	100.00% <ø> (ø)

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