Possible improvements to YAML generation in generate-workflow.py

[mattmccutchen-cci]

Some ideas to make the YAML generation in generate-workflow.py better after #7:

• From John: "The python script could include a call to yamllint to sanity check the generated workflow. Easy enough to run manually, but it might catch trivial errors in future changes." This would require everyone who runs generate-workflow.py to install yamllint, so we'd have to think about whether it's worth it.
• We could generate the output using a YAML library rather than a bunch of manual string manipulation. (Or maybe there is even a library specifically for generating GitHub workflow files?) If there are hard-coded chunks of the workflow file that are easier to understand in YAML format than as hard-coded calls to the YAML library to build the corresponding data structures, we can probably just have the YAML library parse hard-coded YAML strings.