The setlocale() is only done in a rare use case (LC_NUMERIC and
LC_CTYPE have a different encoding), it's safe to call localeconv()
in all other cases.

)

* Add _Py_GetLocaleconvNumeric() function: decode decimal_point and
  thousands_sep fields of localeconv() from the LC_NUMERIC encoding,
  rather than decoding from the LC_CTYPE encoding.
* Modify locale.localeconv() and "n" formatter of str.format() (for
  int, float and complex to use _Py_GetLocaleconvNumeric()
  internally.

(cherry picked from commit cb064fc)