from https://pip.pypa.io/en/stable/news/:

Remove the --build-dir option and aliases, one last time. (#10485)

$ pip3 --version
pip 22.0.2 from /usr/lib/python3/dist-packages/pip (python 3.10)

$ fpm -s python -t deb --python-bin python3 --python-pip pip3 --python-package-name-prefix python3 --python-install-lib=/usr/lib/python3/dist-packages --python-install-bin=/usr/bin --deb-no-default-config-files --verbose flask

fpm ultimately attempts to run the following command when using -s python:

$ pip3 download --no-clean --no-deps --no-binary :all: -i https://pypi.python.org/simple --build /tmp/package-python-build-ca294820df13179a5615a2cfb5e98006916ac6b54d90323c649d9b3cd190/flask flask

Usage:
pip3 download [options] [package-index-options] ...
pip3 download [options] -r [package-index-options] ...
pip3 download [options] ...
pip3 download [options] ...
pip3 download [options] <archive url/path> ...

no such option: --build