Metadata-generation-failed error installing pystan on debian

I’m trying to install pystan on a Raspberry Pi 4.

Running python3 -m pip install pystan yields the following

Looking in indexes:,
Collecting pystan
  Using cached (13 kB)
Collecting aiohttp<4.0,>=3.6 (from pystan)
  Using cached (486 kB)
Collecting clikit<0.7,>=0.6 (from pystan)
  Using cached (91 kB)
INFO: pip is looking at multiple versions of pystan to determine which version is compatible with other requirements. This could take a while.
Collecting pystan
  Using cached (13 kB)
  Using cached (13 kB)
  Using cached (13 kB)
  Using cached (13 kB)
  Using cached (12 kB)
  Using cached (12 kB)
  Using cached pystan- (16.2 MB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... error
  error: subprocess-exited-with-error
  × Preparing metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [1 lines of output]
      Cython>=0.22 and NumPy are required.
      [end of output]
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

This is weird to me because I have both Cython (0.29.34) and Numpy (1.21.6) installed and working fine. I’d be grateful for any advice!