bpo-34589: Add -X coerce_c_locale command line option (GH-9378)

Add a new -X coerce_c_locale command line option to control C locale
coercion (PEP 538).
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index b90366d..a7f2928 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -656,9 +656,8 @@
 
     def c_locale_get_error_handler(self, locale, isolated=False, encoding=None):
         # Force the POSIX locale
-        env = os.environ.copy()
+        env = dict(os.environ)
         env["LC_ALL"] = locale
-        env["PYTHONCOERCECLOCALE"] = "0"
         code = '\n'.join((
             'import sys',
             'def dump(name):',
@@ -668,7 +667,10 @@
             'dump("stdout")',
             'dump("stderr")',
         ))
-        args = [sys.executable, "-X", "utf8=0", "-c", code]
+        args = [sys.executable,
+                "-X", "utf8=0",
+                "-X", "coerce_c_locale=0",
+                "-c", code]
         if isolated:
             args.append("-I")
         if encoding is not None: