[yt-dev] Issue #1053: add_field fails in Python 3 (yt_analysis/yt)

[astrofrog]

New issue 1053: add_field fails in Python 3
https://bitbucket.org/yt_analysis/yt/issues/1053/add_field-fails-in-python-3

astrofrog:

The following example:

```python
import os
from yt import load

ds = load(os.path.join('DD0010', 'moving7_0010'))

def _dust_density(field, data):
    return data["density"].in_units('g/cm**3') * 0.01

ds.add_field("dust_density", function=_dust_density, units='g/cm**3')
```

fails in Python 3 with the following error:

```python
Traceback (most recent call last):
  File "derived.py", line 9, in <module>
    ds.add_field("dust_density", function=_dust_density, units='g/cm**3')
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/yt/data_objects/static_output.py", line 910, in add_field
    deps, _ = self.field_info.check_derived_fields([name])
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/yt/fields/field_info_container.py", line 343, in check_derived_fields
    self.ds.derived_field_list = list(sorted(dfl))
TypeError: unorderable types: str() < tuple()
```

This is with yt 3.2 in Python 3.4. The issue is that in Python 3, strings can no longer be compared to tuples, so the sorting fails.