So that the wrapper library can be installed with `pip install
<archive>`. The setup.py is Windows-specific, but CI only
produces Windows builds so that's not currently an issue.
The __init__.py re-exports the .pyd module so that the .pyd and
.dll can be installed into a subdirectory of site-packages for
neatness, and so that the installed module isn't just to work
around pip install being unable to install a pyd with no package.
