what’s going on?
Ah that’s super weird. That would suggest to me that the Raspberry PI foundations index’s are unaware of spidev and Python 3.9 in the distro you’re using.
You totally nailed it when you were talking about Bullseye and Bookworm having a conflict. 
I found this post online that explains the problem well.
python3-spidevis pulled fromhttps://archive.raspberrypi.org/Bullseye repository, whilepython3is pulled fromhttp://raspbian.raspberrypi.org/Bookworm repository. This is causing a conflict between versions now. Unfortunately there is still no Bookworm repository available fromhttps://archive.raspberrypi.org/
It gets more interesting.
Just last night at 4:35pm Bullseye was updated in the Raspberry PI Index.
This is wild, but it might be that running sudo apt update magics your problem away. Unlikely, but give it a go?
downgrading to python 3.7
Depends: python3 (< 3.8) implies that if the version of python on the system is 3.7 the installer would be chill. You could install it here 
=> Python Release Python 3.7.0 | Python.org
using pip
Python’s personal package manager, pip, doesn’t query the Raspberry Pi Archives, instead querying it’s own rainbow tables of packages. It’s frustrating that the apt-get indexing isn’t working as intended, but if you use Pypi on this occasion you might find Spidev comes down without a problem.
pip install spidev
