This commit finds a problem that has been observed in read code.

Consider the following code:

```
from typing import TYPE_CHECKING
...
if TYPE_CHECKING
    from a import A, B
    from b import C

...
def f() -> "B":
    ...

def f()
    # Oops! C is actually used here.
    C()
```

This commit ignores all code that is guarded by TYPE_CHECKING in
order to find mistakes like this. This constant is always False
when mypy is not running anyway.

This reverts commit a6f0dd2.