Change `vaopy` to `pyvo` in tests

None of the tests work for me:
```
$ python tests/dal/testSIA.py
Traceback (most recent call last):
  File "tests/dal/testSIA.py", line 9, in <module>
    import vaopy.dal.query as dalq
ImportError: No module named vaopy.dal.query
```

I think `vaopy` has to be changed to `pyvo` throughout?