Update requirements.txt (#3551)
Update requirements.txt (#3551)

Fixes compatibility issues introduced by pip 20.3 and its more aggressive package dependencies resolver.
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq