Now it seems to complain about internal packages:

ERROR: Could not find a version that satisfies <interal-package> (from -c constraints.txt (line 1)) (from versions: none)
pid: <PID> -> path/to/python /.../.cache/pants/named_caches/pex_root/pip.pex/<hash> --disable-pip-version-check --isolated --no-python-version-warning --exists-action a -q -cache-dir /.../.cache/pants/named_caches/pex_root download --dest /tmp/<tmp dir>/<path to python bin separated with .> --index-url <https://pypi.org/simple> --extra-index-url <http://our-artifactory>:port/.../simple --trusted-host our-actifactory:port --header Cache-Control:max-age=3600 --retries 5 timeout 15 --constraint constraints.txt <internal-package> flask requests

raised Executing <<the command above>>

• In pants.toml I added

[python-repos]
indexes.add = ["<http://our-artifactory>:port/.../simple"]