fix edge case for humanize month

[yiransii]

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 edge case handling for the humanize method to describe the delta of both 30 days and 31 days as "1 month".

For instance,

dt=arrow.get("2020-11-08T15:12:18.911919+00:00")
later=dt.shift(months=+1)
print(dt.humanize(later))
print(later.humanize(dt))

should print

a month ago
in a month

Closes: #749

[codecov[bot]]

Codecov Report

Merging #894 (0e76c3e) into master (e521f27) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #894   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1817      1819    +2     
  Branches       320       321    +1     
=========================================
+ Hits          1817      1819    +2     

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e521f27...0e76c3e. Read the comment docs.

[systemcatch]

Hey @yiransii I'll close this for now, let me know if you want to take it up again.