Fix str.format(), float.__format__() and complex.__format__() methods
for non-ASCII decimal point when using the "n" formatter.
Rewrite _PyUnicode_InsertThousandsGrouping(): it now requires
a _PyUnicodeWriter object for the buffer and a Python str object
for digits.
(cherry picked from commit 59423e3ddd)
locale.localeconv() now sets temporarily the LC_CTYPE locale to the
LC_MONETARY locale if the two locales are different and monetary
strings are non-ASCII. This temporary change affects other threads.
Changes:
* locale.localeconv() can now set LC_CTYPE to LC_MONETARY to decode
monetary fields.
* Add LocaleInfo.grouping_buffer: copy localeconv() grouping string
since it can be replaced anytime if a different thread calls
localeconv().
(cherry picked from commit 02e6bf7f20)
* 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.
* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.
