don't bother keeping a set we'll never use

Lib/importlib/_bootstrap.py

@@ -842,6 +842,7 @@ class _FileFinder:
         # We store two cached versions, to handle runtime changes of the
         # PYTHONCASEOK environment variable.
         self._path_cache = set(contents)
+        if sys.platform.startswith(CASE_INSENSITIVE_PLATFORMS):
             self._relaxed_path_cache = set(fn.lower() for fn in contents)