Explicitly set default_flow_style to False
In PyYAML 5.1 they changed this default in the library. Because
we weren't explicitly setting it, this broke our unit tests.
Since default_flow_style=False is usually what you want (hence why
PyYAML changed it), let's set it explicitly.
Also note that this was already being set for dump, but not for
dumps. This is also more consistent.