Drop extra IO type properties only needed by Python 3.6

Python 3.7 is now our minimum-supported Python, and so we don't need to add
these properties any more.

See #2566.

tests/test_null_file.py

@@ -5,9 +5,6 @@ def test_null_file():
     file = NullFile()
     with file:
         assert file.write("abc") == 0
-        assert file.mode == ""
-        assert file.name == "NullFile"
-        assert not file.closed()
         assert file.close() is None
         assert not file.isatty()
         assert file.read() == ""