Issue #14581: Windows users are allowed to import modules w/o taking

the file suffix's case into account, even when doing a case-sensitive
import.

Lib/importlib/_bootstrap.py

@@ -841,7 +841,23 @@ class _FileFinder:
         contents = _os.listdir(path)
         # We store two cached versions, to handle runtime changes of the
         # PYTHONCASEOK environment variable.
+        if not sys.platform.startswith('win'):
             self._path_cache = set(contents)
+        else:
+            # Windows users can import modules with case-insensitive file
+            # suffixes (for legacy reasons). Make the suffix lowercase here
+            # so it's done once instead of for every import. This is safe as
+            # the specified suffixes to check against are always specified in a
+            # case-sensitive manner.
+            lower_suffix_contents = set()
+            for item in contents:
+                name, dot, suffix = item.partition('.')
+                if dot:
+                    new_name = '{}.{}'.format(name, suffix.lower())
+                else:
+                    new_name = name
+                lower_suffix_contents.add(new_name)
+            self._path_cache = lower_suffix_contents
         if sys.platform.startswith(CASE_INSENSITIVE_PLATFORMS):
             self._relaxed_path_cache = set(fn.lower() for fn in contents)