Add support for py.test unit-testing on machines with no device

```
ImportError while importing test module 

/Users/aphearin/work/repositories/python/chopperhack19/chopperhack19/mock_obs/tests/test_gaussian_weighted_pair_counts.py

Traceback:
chopperhack19/mock_obs/tests/test_gaussian_weighted_pair_counts.py:8: in <module>
    from .. import (
E   ImportError: cannot import name 'count_weighted_pairs_3d_cuda_smem'
```