Revert "bpo-38691 Added a switch to ignore PYTHONCASEOK when -E or -I flags passed (#18314)" (GH-18553)

This reverts commit d83b660.

Author: vstinner

Doc/library/functions.rst

@@ -1829,9 +1829,6 @@ are always available.  They are listed here in alphabetical order.
       Negative values for *level* are no longer supported (which also changes
       the default value to 0).
 
-   .. versionchanged:: 3.9
-      When the command line options :option:`-E` or :option:`-I` are being used,
-      the environment variable :envvar:`PYTHONCASEOK` is now ignored.
 
 .. rubric:: Footnotes
 

Doc/whatsnew/3.9.rst

@@ -584,9 +584,6 @@ Changes in the Python API
   since the *buffering* parameter has been removed.
   (Contributed by Victor Stinner in :issue:`39357`.)
 
-* The :mod:`importlib` module now ignores the :envvar:`PYTHONCASEOK`
-  environment variable when the :option:`-E` or :option:`-I` command line
-  options are being used.
 
 CPython bytecode changes
 ------------------------

Lib/importlib/_bootstrap_external.py

@@ -35,7 +35,7 @@ def _make_relax_case():
 
         def _relax_case():
             """True if filenames must be checked case-insensitively."""
-            return not sys.flags.ignore_environment and key in _os.environ
+            return key in _os.environ
     else:
         def _relax_case():
             """True if filenames must be checked case-insensitively."""