Add explicit bounds to pyyaml

When using pip install --pre ., the installation fails because dask requires pyyaml without upper bounds, which installs the broken pyyamml==6.0.2rc1 release.

There is pyyamml!=6.0.2rc1 requirement for ewokscore but not sure setuptools/pip are able to reconcile sub-dependencies synchronously. I guess it does things sequentially so that if dask dependencies gets resolved first, then tough luck, you end up with the pyyaml==6.0.2rc1.

If all goes well, it should install another version of pyyaml when reaching the ewokscore install step but it breaks before that.

I think the best way to fix this for now is to explicitly set the bound of pyyaml in this project.