Fix output of --list-locales to not use bytes reprs

[morganwahl]

This adds a unit test for the --list-locales argument, and updates its implementation to not output bytes reprs.

In other words, goes from this:

...
b'fr_CH           French (Switzerland)'
b'fr_CI           French (C\xc3\xb4te d\xe2\x80\x99Ivoire)'
b'fr_CM           French (Cameroon)'
...

to this

...
fr_CH           French (Switzerland)
fr_CI           French (Côte d’Ivoire)
fr_CM           French (Cameroon)
...

[codecov]

Codecov Report

Merging #845 (a805eb7) into master (6be6b1f) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
+ Coverage   91.09%   91.30%   +0.21%     
==========================================
  Files          23       23              
  Lines        4156     4154       -2     
==========================================
+ Hits         3786     3793       +7     
+ Misses        370      361       -9     

Impacted Files	Coverage Δ
babel/messages/frontend.py	87.79% <100.00%> (+1.69%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[akx]

Thanks for the contribution! I think we can simplify things a bunch :)

[akx]

I folded in my review comments. Thank you for the contribution!