subshape avoids producing vars on slice inputs


### Before

```Python
In [1]: from datashape import dshape

In [2]: ds = dshape('5 * 3 * int32')

In [3]: ds.subshape[2:, 0]
Out[3]: dshape("var * int32")
```

### After

```Python
In [1]: from datashape import dshape

In [2]: ds = dshape('5 * 3 * int32')

In [3]: ds.subshape[2:, 0]
Out[3]: dshape("3 * int32")
```