Improve handling for locale=None

[akx]

This PR improves the handling of None as a locale= argument.

Namely:

• The error message Locale.parse() raises for falsy arguments is more descriptive.
• Explicitly passing in None for a public API means "use the default", not "crash on me".
  • ⚠️ 👉 This could change the behavior of user code that is not very well defined to begin with, that would have expected an exception to be raised for a None locale, but I hope that sort of code isn't very popular.
  • If a default is not available (e.g. on Windows, or when environment variables are fubar'd), we'll still crash, but with a nicer error message than ever before.

Fixes #1156.
Closes #1158 (supersedes it).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.31%. Comparing base (6bbdc0e) to head (9327e5f).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1164   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files          27       27           
  Lines        4638     4654   +16     
=======================================
+ Hits         4235     4250   +15     
- Misses        403      404    +1     

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[tomasr8]

I'll try to take a look later today :)

[tomasr8]

Nice! I left you some comments :)
Hopefully I didn't miss anything with so many similar changes 😅

[tomasr8]

LGTM, just one thing: