Alternate PR is up at https://github.com/python/cpython/pull/9257

There's one setting that remains in CoreConfig: "warn_on_c_locale", which _Py_UnixMain also uses to decide whether or not to actually emit the passed in locale coercion warning.

The coercion warning information itself moves out of CoreConfig, and into the private _PyMain helper struct.

The signature of _Py_CoerceLegacyLocale changed to both return an integer indicating whether or not coercion took place, as well as supporting two output variables that allow the display of the coercion warning message to be deferred (or skipped entirely).