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.

Lib/test/test_sys.py

@@ -527,7 +527,7 @@ class SysModuleTest(unittest.TestCase):
                  "inspect", "interactive", "optimize", "dont_write_bytecode",
                  "no_user_site", "no_site", "ignore_environment", "verbose",
                  "bytes_warning", "quiet", "hash_randomization", "isolated",
-                 "dev_mode")
+                 "dev_mode", "utf8_mode")
         for attr in attrs:
             self.assertTrue(hasattr(sys.flags, attr), attr)
             attr_type = bool if attr == "dev_mode" else int
@@ -535,6 +535,8 @@ class SysModuleTest(unittest.TestCase):
         self.assertTrue(repr(sys.flags))
         self.assertEqual(len(sys.flags), len(attrs))
 
+        self.assertIn(sys.flags.utf8_mode, {0, 1, 2})
+
     def assert_raise_on_new_sys_type(self, sys_attr):
         # Users are intentionally prevented from creating new instances of
         # sys.flags, sys.version_info, and sys.getwindowsversion.
@@ -710,8 +712,8 @@ class SysModuleTest(unittest.TestCase):
         # have no any effect
         out = self.c_locale_get_error_handler(encoding=':')
         self.assertEqual(out,
-                         'stdin: surrogateescape\n'
-                         'stdout: surrogateescape\n'
+                         'stdin: strict\n'
+                         'stdout: strict\n'
                          'stderr: backslashreplace\n')
         out = self.c_locale_get_error_handler(encoding='')
         self.assertEqual(out,