Added week and weeks timeframes for Czech locale

[mesemus]

Pull Request Checklist

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

• [ x ] 🧪 Added tests for changed code.
• [ x ] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [ x ] 🧹 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.

Description of Changes

Added week and weeks timeframes to the czech locale. Documentation is not affected.

Closes: #815

[codecov-commenter]

Codecov Report

Merging #816 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #816   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1757      1757           
  Branches       300       300           
=========================================
  Hits          1757      1757           

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

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[systemcatch]

Hi @mesemus, thank you for the PR. I'm happy with this but will give Kris a chance to chime in.