bpo-41401: Fix test_fspath_support in test_io. (GH-21640)

The error is exposed on non-UTF-8 locales.

Author: serhiy-storchaka

Lib/test/test_io.py

@@ -928,7 +928,7 @@ def check_path_succeeds(path):
                 self.assertEqual(f.read(), "egg\n")
 
         check_path_succeeds(FakePath(os_helper.TESTFN))
-        check_path_succeeds(FakePath(os_helper.TESTFN.encode('utf-8')))
+        check_path_succeeds(FakePath(os.fsencode(os_helper.TESTFN)))
 
         with self.open(os_helper.TESTFN, "w") as f:
             bad_path = FakePath(f.fileno())