Incompatible requirements for `protobuf` in Cirq

[thisac]

Cirq has the requirement google-api-core[grpc] >= 1.14.0, < 2.0.0dev as well as protobuf==3.8.0 (as of the latest relase; protobuf >= 3.12.0, in master), while the google-api-core recently updated its requirement to protobuf >= 3.12.0 as of v.1.21.0. An error will thus occur if using the latest version of google-api-core, which can be fixed by simply installing an older version of google-api-core.

See related Cirq issue at quantumlib/Cirq#3110.