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 Textualize#2566.

rich/_null_file.py

@@ -4,19 +4,6 @@
 
 class NullFile(IO[str]):
 
-    # TODO: "mode", "name" and "closed" are only required for Python 3.6.
-
-    @property
-    def mode(self) -> str:
-        return ""
-
-    @property
-    def name(self) -> str:
-        return "NullFile"
-
-    def closed(self) -> bool:
-        return False
-
     def close(self) -> None:
         pass
 

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() == ""