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/Modules/clinic/_winapi.c.h b/Modules/clinic/_winapi.c.h
index 01bba36..e5781efb 100644
--- a/Modules/clinic/_winapi.c.h
+++ b/Modules/clinic/_winapi.c.h
@@ -889,4 +889,22 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=fba2ad7bf1a87e4a input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(_winapi_GetACP__doc__,
+"GetACP($module, /)\n"
+"--\n"
+"\n"
+"Get the current Windows ANSI code page identifier.");
+
+#define _WINAPI_GETACP_METHODDEF    \
+    {"GetACP", (PyCFunction)_winapi_GetACP, METH_NOARGS, _winapi_GetACP__doc__},
+
+static PyObject *
+_winapi_GetACP_impl(PyObject *module);
+
+static PyObject *
+_winapi_GetACP(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return _winapi_GetACP_impl(module);
+}
+/*[clinic end generated code: output=fd91c1ec286f0bf3 input=a9049054013a1b77]*/