CLDR 37 support

[akx]

This PR adds support for CLDR 37 (published 2020-04).

No new features (prefix compound patterns, grammatical forms) are supported at this time.

It also cleans up some deprecation warnings in certain tests.

[codecov-commenter]

Codecov Report

Merging #734 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #734      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.01%     
==========================================
  Files          24       24              
  Lines        4186     4126      -60     
==========================================
- Hits         3809     3754      -55     
+ Misses        377      372       -5     

Impacted Files	Coverage Δ
babel/support.py	83.14% <ø> (-0.07%)	⬇️
babel/units.py	100.00% <100.00%> (ø)
babel/core.py	95.51% <0.00%> (-0.54%)	⬇️
babel/messages/catalog.py	95.56% <0.00%> (-0.08%)	⬇️
babel/numbers.py	97.98% <0.00%> (+0.27%)	⬆️
babel/plural.py	96.38% <0.00%> (+0.31%)	⬆️
babel/dates.py	91.93% <0.00%> (+0.32%)	⬆️

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