Process: follow numpy's python support cadence

[balopat]

Is your feature request related to a use case or problem? Please describe.

We need clarity around what python versions we support. As python evolves, we get issues for incompatibility. (see issues labeled with python3.8), and we should have CI tests for the supported python versions.

Describe the solution you'd like

As we depend on NumPy, we could follow their support process of 42 months window: https://numpy.org/neps/nep-0029-deprecation_policy.html

What is the urgency from your perspective for this issue? Is it blocking important work?

P2 - we should do it in the next couple of quarters

[balopat]

No objections at Cirq Cynq!