BUG: Use astimezone instead of tz_convert.

Makes get_datetime agnostic as to whether its input is a datetime or a
Timestamp.  (astimezone is an alias to tz_convert when the input is a
Timestamp).
Travis' Python 3 build failed on this line:
```
zipline/algorithm.py:787:31: F821 undefined name 'basestring'
```

Maybe use `six.string_types`?