First you should decide what is your particular purpose. The official Python documentation on extending and embedding the Python interpreter was mentioned above, I can add a good overview of binary extensions. The use cases can be divided into 3 categories:
- accelerator modules: to run faster than the equivalent pure Python code runs in CPython.
- wrapper modules: to expose existing C interfaces to Python code.
- low level system access: to access lower level features of the CPython runtime, the operating system, or the underlying hardware.
In order to give some broader perspective for other interested and since your initial question is a bit vague ("to a C or C++ library") I think this information might be interesting to you. On the link above you can read on disadvantages of using binary extensions and its alternatives.
Apart from the other answers suggested, if you want an accelerator module, you can try Numba. It works "by generating optimized machine code using the LLVM compiler infrastructure at import time, runtime, or statically (using the included pycc tool)".
