Only a single variable in queries can be currently requested

[Test `test_complex_queries`](https://github.com/severin-lemaignan/minimalkb/blob/master/testing/test.py#L183) expects that several variable can be queried at once, and accordingly fails.

This enhancement should take place [here](https://github.com/severin-lemaignan/minimalkb/blob/master/src/minimalkb/backends/sqlite_queries.py).
