Pin protobouf that breaks TensorBoard in PyTorch
# What does this PR do?

The recent release of Protobuf (4.21) has broken TensorBoard in PyTorch and thus multiple tests. This PR pins protobuf to fix said tests.
Yes to rebuliding a docker image ASAP. I will merge this once all tests pass :-) <|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Python dependencies 🤦‍♀️