bpo-29240: PEP 540: Add a new UTF-8 Mode (#855)

* Add -X utf8 command line option, PYTHONUTF8 environment variable
  and a new sys.flags.utf8_mode flag.
* If the LC_CTYPE locale is "C" at startup: enable automatically the
  UTF-8 mode.
* Add _winapi.GetACP(). encodings._alias_mbcs() now calls
  _winapi.GetACP() to get the ANSI code page
* locale.getpreferredencoding() now returns 'UTF-8' in the UTF-8
  mode. As a side effect, open() now uses the UTF-8 encoding by
  default in this mode.
* Py_DecodeLocale() and Py_EncodeLocale() now use the UTF-8 encoding
  in the UTF-8 Mode.
* Update subprocess._args_from_interpreter_flags() to handle -X utf8
* Skip some tests relying on the current locale if the UTF-8 mode is
  enabled.
* Add test_utf8mode.py.
* _Py_DecodeUTF8_surrogateescape() gets a new optional parameter to
  return also the length (number of wide characters).
* pymain_get_global_config() and pymain_set_global_config() now
  always copy flag values, rather than only copying if the new value
  is greater than the old value.
diff --git a/Doc/c-api/sys.rst b/Doc/c-api/sys.rst
index 95d9d65..20bc7bd 100644
--- a/Doc/c-api/sys.rst
+++ b/Doc/c-api/sys.rst
@@ -127,6 +127,9 @@
 
    .. versionadded:: 3.5
 
+   .. versionchanged:: 3.7
+      The function now uses the UTF-8 encoding in the UTF-8 mode.
+
 
 .. c:function:: char* Py_EncodeLocale(const wchar_t *text, size_t *error_pos)
 
@@ -138,12 +141,15 @@
    to free the memory. Return ``NULL`` on encoding error or memory allocation
    error
 
-   If error_pos is not ``NULL``, ``*error_pos`` is set to the index of the
-   invalid character on encoding error, or set to ``(size_t)-1`` otherwise.
+   If error_pos is not ``NULL``, ``*error_pos`` is set to ``(size_t)-1`` on
+   success,  or set to the index of the invalid character on encoding error.
 
    Use the :c:func:`Py_DecodeLocale` function to decode the bytes string back
    to a wide character string.
 
+   .. versionchanged:: 3.7
+      The function now uses the UTF-8 encoding in the UTF-8 mode.
+
    .. seealso::
 
       The :c:func:`PyUnicode_EncodeFSDefault` and
@@ -151,6 +157,9 @@
 
    .. versionadded:: 3.5
 
+   .. versionchanged:: 3.7
+      The function now supports the UTF-8 mode.
+
 
 .. _systemfunctions: