Issue #13706: Fix format(int, "n") for locale with non-ASCII thousands separator

* Decode thousands separator and decimal point using PyUnicode_DecodeLocale()
   (from the locale encoding), instead of decoding them implicitly from latin1
 * Remove _PyUnicode_InsertThousandsGroupingLocale(), it was not used
 * Change _PyUnicode_InsertThousandsGrouping() API to return the maximum
   character if unicode is NULL
 * Replace MIN/MAX macros by Py_MIN/Py_MAX
 * stringlib/undef.h undefines STRINGLIB_IS_UNICODE
 * stringlib/localeutil.h only supports Unicode