Utilize __init_subclass__ instead of inspect on Locale mapping to improve type checking

[isac322]

Pull Request Checklist

• 🧪 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.

Description of Changes

Python 3.6 introduced __init_subclass__ that invoked when any subclass is declared. It can replace behavior of arrow.locales._map_locales().

Additional feature

__init_subclass__ invoked on every subclassing -> Subclass of Locale outside of arrow package also registered on arrow.locales._locale_map.

Example

>>> from arrow.locales import Locale
>>> class Locale1(Locale):
...   names = ['asdf']
... 
>>> from arrow import locales
>>> locales.get_locale('asdf')
<__main__.Locale1 object at 0x100f19b50>

[codecov]

Codecov Report

Merging #920 (44cd011) into master (485d40b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #920      +/-   ##
==========================================
- Coverage   99.74%   99.74%   -0.01%     
==========================================
  Files          10       10              
  Lines        1937     1934       -3     
  Branches      313      312       -1     
==========================================
- Hits         1932     1929       -3     
  Misses          4        4              
  Partials        1        1              

Impacted Files	Coverage Δ
arrow/locales.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 485d40b...44cd011. Read the comment docs.

[krisfremen]

One comment from me, otherwise, I don't see anything wrong from a first glance.

Will take a better look later today.

[jadchaar]

LGTM but had a small comment

[jadchaar]

Should we maintain the locale_name.lower() like in the old code?