bytes formatting incorrect in python 3

The following runs properly:

```
def foo(a: bytes, b:bytes) -> bytes:
    return b'%s:%s' % (a, b)
assert foo(b'a', b'b') == b'a:b'
```

However mypy complains: 

```
example.py: note: In function "foo":
example.py:2: error: Incompatible return value type (got "str", expected "bytes")
```
