bpo-38691: importlib ignores PYTHONCASEOK if -E is used (GH-18627)

The importlib module now ignores the PYTHONCASEOK
environment variable when the -E or -I command line
options are being used.

Lib/importlib/_bootstrap_external.py

@@ -34,8 +34,8 @@ def _make_relax_case():
             key = b'PYTHONCASEOK'
 
         def _relax_case():
-            """True if filenames must be checked case-insensitively."""
-            return key in _os.environ
+            """True if filenames must be checked case-insensitively and ignore environment flags are not set."""
+            return not sys.flags.ignore_environment and key in _os.environ
     else:
         def _relax_case():
             """True if filenames must be checked case-insensitively."""