fix python3 bytes/str bug in Person

`__str__()` should return a type `str()` not `bytes()`

python3:
```
from catkin_pkg.package import Person
p=Person('plop')
str(p)
```

error:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type bytes)
```