bpo-28180: Implementation for PEP 538 (#659)

- new PYTHONCOERCECLOCALE config setting
- coerces legacy C locale to C.UTF-8, C.utf8 or UTF-8 by default
- always uses C.UTF-8 on Android
- uses `surrogateescape` on stdin and stdout in the coercion
  target locales
- configure option to disable locale coercion at build time
- configure option to disable C locale warning at build time
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index ed78e2a..3844812 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -682,6 +682,7 @@
         # Force the POSIX locale
         env = os.environ.copy()
         env["LC_ALL"] = "C"
+        env["PYTHONCOERCECLOCALE"] = "0"
         code = '\n'.join((
             'import sys',
             'def dump(name):',