Standardize locales and improve locale validation

[jadchaar]

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

Closes: #952

[codecov]

Codecov Report

Merging #954 (989d93c) into master (ac6a204) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #954   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          10       10           
  Lines        1957     1964    +7     
  Branches      313      313           
=======================================
+ Hits         1952     1959    +7     
  Misses          4        4           
  Partials        1        1           

Impacted Files	Coverage Δ
arrow/api.py	100.00% <100.00%> (ø)
arrow/arrow.py	99.00% <100.00%> (+<0.01%)	⬆️
arrow/constants.py	100.00% <100.00%> (ø)
arrow/factory.py	100.00% <100.00%> (ø)
arrow/formatter.py	100.00% <100.00%> (ø)
arrow/locales.py	100.00% <100.00%> (ø)
arrow/parser.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 ac6a204...989d93c. Read the comment docs.

[jadchaar]

@systemcatch @krisfremen do ya'll think I should change the variable that is referenced when we throw errors about locales? E.g. duplicate locale registered in a subclass or if a locale does not exist in a call to get_locale? Currently, we use the original locale they pass in like en_US, but if we are transforming it into en-us for our internal lookup, should we display this in the error message instead?

[krisfremen]

Since we are doing a bit of sanitizing anyway, I think what we have in the locales should be fine to return in the errors, it'll help to adopt the new codes also.

[systemcatch]

I agree with Kris, let's start returning the new codes in errors.

[jadchaar]

Fixed @krisfremen @systemcatch!

[krisfremen]

One question from first glance, otherwise looking good.

[krisfremen]

One more thought off the top of my head, but I'll have to double check this.

[krisfremen]

Looks good to me. 👍

[jadchaar]

Look good to you @systemcatch?

[systemcatch]

Yeah Jad looks good to merge.