Python is installed in a local directory.
My directory tree looks like this:
(local directory)/site-packages/toolkit/interface.py
In my case, the problem was I was linking to debug python & boost::Python, which requires that the extension be FooLib_d.pyd, not just FooLib.pyd; renaming the file or updating CMakeLists.txt properties fixed the error.
python
boost::Python
FooLib_d.pyd
FooLib.pyd
CMakeLists.txt