consistent str of datashapes with timezone info

Makes `__str__` of datashapes with timezone information consistent
between Python 2 and Python 3.

Before (in Python 2.x):
```
>>> str(datashape.dshape("datetime[tz='UTC']"))
"datetime[tz=u'UTC']"
```

After:
```
>>> str(datashape.dshape("datetime[tz='UTC']"))
"datetime[tz='UTC']"
```

Before and After in Python 3.x:
```
>>> str(datashape.dshape("datetime[tz='UTC']"))
"datetime[tz='UTC']"
```