Issue #16826: Don't check for PYTHONCASEOK when using -E.

This commit fixes a regression that sneaked into Python 3.3 where importlib
was not respecting -E when checking for the PYTHONCASEOK environment variable.

Lib/test/test_importlib/extension/test_case_sensitivity.py

@@ -5,7 +5,8 @@
 from importlib import _bootstrap
 from .. import util
 from . import util as ext_util
-
+import os
+import subprocess
 
 @util.case_insensitive_tests
 class ExtensionModuleCaseSensitivityTest(unittest.TestCase):
@@ -29,14 +30,34 @@ def test_case_sensitive(self):
             self.assertIsNone(loader)
 
     def test_case_insensitivity(self):
-        with support.EnvironmentVarGuard() as env:
-            env.set('PYTHONCASEOK', '1')
-            if b'PYTHONCASEOK' not in _bootstrap._os.environ:
-                self.skipTest('os.environ changes not reflected in '
-                              '_os.environ')
-            loader = self.find_module()
-            self.assertTrue(hasattr(loader, 'load_module'))
+        find_snippet = """if True:
+            from importlib import _bootstrap
+            import sys
+            finder = _bootstrap.FileFinder('{path}',
+                                           (_bootstrap.ExtensionFileLoader,
+                                            _bootstrap.EXTENSION_SUFFIXES))
+            loader = finder.find_module('{bad_name}')
+            print(str(hasattr(loader, 'load_module')))
+            """.format(bad_name=ext_util.NAME.upper(), path=ext_util.PATH)
 
+        newenv = os.environ.copy()
+        newenv["PYTHONCASEOK"] = "1"
+
+        def check_output(expected, extra_arg=None):
+            args = [sys.executable]
+            if extra_arg:
+                args.append(extra_arg)
+            args.extend(["-c", find_snippet])
+            p = subprocess.Popen(args, stdout=subprocess.PIPE, env=newenv)
+            actual = p.communicate()[0].decode().strip()
+            self.assertEqual(expected, actual)
+            self.assertEqual(p.wait(), 0)
+
+        # Test with PYTHONCASEOK=1.
+        check_output("True")
+
+        # Test with PYTHONCASEOK=1 ignored because of -E.
+        check_output("False", "-E")