Update rosdep CI to enforce compatibility with older PyYAML

[cottsay]

Somewhere between 3.13 and 5.3.1, PyYAML started supporting this syntax without any quotes:

foo: [bar:baz]

Some of our older platforms don't have a new enough version of PyYAML to parse that syntax. We should enforce compatibility in CI so that we don't break those platforms, at least until they're EOL.