Maybe MYPY, PY2 and PY3 can be imported from typing instead of having to set them?

I have to write code like this:

```
MYPY = False
if MYPY:
   # do mypy stuff
```

I think it would be more elegant if I could write

```
from typing import MYPY
```

instead. That way the values can actually be defined properly in the stubs and lib-typing files.
