Add PyYAML as a dependency of omero-py

[sbesson]

In the current construction of omero-py, there are two public APIs which require PyYAML at runtime: the CLI import --bulk functionalitity as well as the omero.util.pydict_text_io utility consumed by a few CLI plugins (render, metadata) for loading YAML files.

497f1bd makes PyYAML a mandatory dependency of omero-py
dcde6b2 also removes the optional import yaml handling and replaces usage of the deprecated yaml.load and yaml.load_all by yaml.safe_load and yaml.safe_load_all

[manics]

In case you missed it sbesson#1

[joshmoore]

The changes all look good to me. Testing today was all green (and travis is happy). Merging.

[sbesson]

I realize I haven't discussed versioning. My inclination would be to release by incrementing the patch version i.e. 5.7.2 as the API is not modified and the runtime dependency exception has always felt as a bug to me.

[joshmoore]

I was thinking about that. I don't imagine adding a dependency, now that it happens transitively, requires a minor bump. Though now that I think about it, I wonder if anything else like https://github.com/ome/conda-omero-py/blob/master/meta.yaml needs a bump, too. We probably need to codify the steps on a requirement addition/modification.