'in' operator rejected with union right operand

Mypy generates a false positive for this fragment:

```py
from typing import Iterable, Container, Union

i: Iterable[str]
c: Container[str]
u: Union[Iterable[str], Container[str]]

'x' in i
'x' in c
'x' in u  # Unsupported right operand type for in ("Union[Iterable[str], Container[str]]")
```

This should be easy to fix.