This is not supported.
Using PortAudio with the Bela audio engine is in theory doable, I think. It would require writing the appropriate
hostapi for PortAudio, that is: re-write something like this (currently for ALSA) to use the Bela API instead. In fairness, it would probably much simpler and shorter than that file, given that Bela gives way less options than ALSA does.
In practice, whether the method above can actually be used in a given context is strictly dependent on how the application that uses PortAudio is written, so you may find yourself having to edit the code of that application as well (e.g: this in the case of pyaudio).
Now, I know nothing about
python, but have you considered using Olivier Belanger's [pyo] (https://github.com/belangeo/pyo) module? Olivier also provides an example project to use with Bela.