Merge branch 'master' into v0.11.0-dev

Merge branch 'master' of https://github.com/quantumlib/cirq into v0.1…

…0.0-dev

Switch to black for formatting

remove accidental print (quantumlib#3384)

releasing v0.9.0

Bump protobuf requirement to 3.12.x (quantumlib#3132)

Fixes quantumlib#3110.

Set version to 0.8.1 on release branch.

Remove dev version on release branch

Update master to 0.8.0.dev (quantumlib#2686)

Backport "Fix doc build by removing tensorflow dependency" onto v0.6 (q…

…uantumlib#2612)

- This is a cherry-pick from quantumlib#2610