Python's ctypes support is broken on macOS Big Sur. This currently affects nixpkgs. Here's an example with python3.8-regex:

https://gist.github.com/dhess/e1b2662a6732618b252096e0de3a04d4

This has been fixed in CPython trunk and backported to 3.9:

python/cpython#22855
python/cpython#23295

Unfortunately, it's not yet been backported to 3.8, which is the default python3Packages set in nixpkgs.