Pin cirq_rigetti pyquil dependency to pyquil-3.*

[pavoljuhas]

Problem: pip install cirq --pre may install pyquil-4.0.0rcN which is
incompatible with cirq_rigetti.

Solution: pin the pyquil dependency to "3.*".

[tanujkhattar]

Do you know what's causing the incompatibility? We should also reach out to cirq-rigetti maintainers and see if they can fix the dependency issue.

I've also run into a few issues like rigetti/qcs-api-client-python#9 that causes introduces transitive constraints for us which seem unnecessary.

cc @dabacon @dstrain115 Can we reach out to a cirq-rigetti owner and ask for a comment? Maybe Josh?

[pavoljuhas]

This should also unblock our nightly builds of API docs, which currently fail on inconsistent installation of cirq_rigetti.

[pavoljuhas]

Do you know what's causing the incompatibility?

pyquil-4.0.0rcN uses qcs-sdk-python instead of qcs-api-client which is assumed by cirq_rigetti.
While pyquil has no stable release 4, pip install --pre ... enables its rcN versions.

Can we reach out to a cirq-rigetti owner and ask for a comment?

That's a good idea to ask them to adjust cirq_rigetti for the upcoming pyquil-4.