[Python-ideas] Making TYPE_CHECKING builtin.

Hi, all.

I want to write type hints without worrying about runtime overhead. Current best practice is:

``` from __future__ import annotations

import typing

if typing.TYPE_CHECKING: import xxx # modules used only in type hints. ```

But it would be nice if I can avoid importing even "typing" module. How about adding TYPE_CHECKING builtin that is False?

``` from __future__ import annotations

if TYPE_CHECKING: from typing import Any, Optional

# We can use Any, Optional, etc here. ```

I wonder if we can make TYPE_CHECKING constant like True, False, and None. But it will break existing `from typing import TYPE_CHECKING` codes.

Regards,