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.
• [x] ⏩ 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.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1157 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 10 10 Lines 2326 2326 Branches 400 400 ========================================= Hits 2326 2326 ``` | [Files](https://app.codecov.io/gh/arrow-py/arrow/pull/1157?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py) | Coverage Δ | | |---|---|---| | [arrow/locales.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1157?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvbG9jYWxlcy5weQ==) | `100.00% <ø> (ø)` | |

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