Partial Humanize Refactor

[anishnya]

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.
• [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.

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Description of Changes

I've completed a partial refactor of Humanize as per #749. This doesn't fix the issues we are having with leap years, or other correctness related issues (that will hopefully come at a later date). This PR cleans up some of the more nasty if statements we had in there, making humanize much shorter, and in my opinion much more readable.

[codecov[bot]]

Codecov Report

Merging #971 (cfee46d) into master (06fe693) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #971   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2036      2009   -27     
  Branches       328       316   -12     
=========================================
- Hits          2036      2009   -27     

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

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