It seems that we don't have a clear policy on how to handle requirements for "optional" modules.

For example #3532 introduced a pass that requieres z3-solver an that was added as a requirements-dev https://github.com/Qiskit/qiskit-terra/pull/3532/files#diff-f4c265073ea4274a80eca85440875e56, while #3043 also introduced a pass with an extra dependency (python-constraint in this case) but did not extend requirements-dev.

What should be the policy here?