Lower coverage requirement to 99% until humanize xfail month tests are fixed

[systemcatch]

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.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Just one digit makes all the difference.

[codecov]

Codecov Report

Merging #937 (d4d2053) into master (fc2cbaa) will decrease coverage by 0.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##            master     #937      +/-   ##
===========================================
- Coverage   100.00%   99.73%   -0.27%     
===========================================
  Files           10       10              
  Lines         1896     1896              
  Branches       311      311              
===========================================
- Hits          1896     1891       -5     
- Misses           0        4       +4     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
arrow/locales.py	100.00% <ø> (ø)
arrow/arrow.py	98.99% <0.00%> (-1.01%)	⬇️

---

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 47c715d...545319d. Read the comment docs.

[jadchaar]

LGTM.

We should revert this once #749 is fixed.