mac/focal yaml serialization differs from bionic's

[ggould-tri]

For whatever reason, yaml serialization on mac and focal produces vertically formatted lists, while serialization on bionic produces horizontally formatted ones. This produces errors like

  example:
+   controller_params: [5.0, 50.0, 5.0, 1000.0]
+   initial_state: [1.2, 0.0, 0.0, 0.0]
-   controller_params:
-   - 5.0
-   - 50.0
-   - 5.0
-   - 1000.0
-   initial_state:
-   - 1.2
-   - 0.0
-   - 0.0
-   - 0.0
    t_final: 30.0
    tape_period: 0.05

The relevant test was nerfed in #13538 but should be rewritten to compare the parsed (or post-parse serialized) rather than a raw reference serialized form in its roundtrip tests (ie, to insist on semantic rather than syntactic equality).

[jwnimmer-tri]

Ideally, the I/O code could be set to use the flow style during serialization, because it's nicer to read. We do that for C++ already:

drake/common/yaml/yaml_write_archive.cc

Lines 29 to 37 in e55b6dd

	case YAML::NodeType::Sequence: {
	// If all children are scalars, then format this sequence onto a
	// single line; otherwise, format as a bulleted list.
	auto style = YAML::EmitterStyle::Flow;
	for (const auto& child : node) {
	if (child.IsSequence() || child.IsMap()) {
	style = YAML::EmitterStyle::Block;
	}
	}

[ggould-tri]

Amen on the nicer-to-read.
I believe we may have ended up on opposite sides of yaml/pyyaml#256 and so must always set default_flow_style when we call the dumper.

[jwnimmer-tri]

Ooh, a tribool found in the wild. Get out the documentary cameras.