Tests are being installed

I noticed that the tests are being installed into site-packages, but I normally expect it to be available in the source for pytest-ing them. Should it be removed?

This can be confirmed in version 21.0.0, latest available in PyPI, with:

```python
import hyperlink
import pathlib
print(list((pathlib.Path(hyperlink.__file__).parent / 'test').glob('*')))
```