DataDescriptor.py[...] -> DataDescriptor[...]

Issue: this promotes python types as the default itemaccess type
Benefit: .py was confusing because it looked like a file extension

```Python
In [1]: from blaze import *
In [2]: csv = CSV('/home/mrocklin/data/hmda/hmda_lar-2012.csv')
```

Before:
```Python
In [3]: csv.py[0]
Out[3]:  ...
```

After:
```Python
In [3]: csv[0]
Out[3]:  ...
```

cc @quasiben @chdoig