Fix shutil.which() test for issue #16993.

Doc/library/shutil.rst

@@ -335,7 +335,7 @@ Directory and files operations
    directories.  For example, on Windows::
 
       >>> shutil.which("python")
-      'C:\\Python33\\python.exe'
+      'C:\\Python33\\python.EXE'
 
    .. versionadded:: 3.3
 

Lib/test/test_shutil.py

@@ -1318,7 +1318,7 @@ class TestWhich(unittest.TestCase):
         # Ask for the file without the ".exe" extension, then ensure that
         # it gets found properly with the extension.
         rv = shutil.which(self.temp_file.name[:-4], path=self.dir)
-        self.assertEqual(rv, self.temp_file.name[:-4] + ".exe")
+        self.assertEqual(rv, self.temp_file.name[:-4] + ".EXE")
 
 
 class TestMove(unittest.TestCase):