i64 could always be true

```
from mypy_extensions import i64

i: i64 = 0
if i: pass
```

Type checked with mypy, this reports:
```
error: has tpe "i64" which does not implement __bool__ or __len__ so it could always be true in boolean context
```

In reality, truthy tests work fine in both compiled and non-compiled code, so seems like there's something missing here for mypy to understand the type correctly.