Fix warnings from flake8-comprehensions

[Zac-HD]

flake8-comprehensions is a lovely plugin to warn about inefficient creation of and operations on collections - and each of the warnings suggests a simple rewrite rule to resolve them. I'm not sure if this is really in scope for autoflake, but thought it worth suggesting an optional flag to do so 😄

[myint]

Thanks for pointing out flake8-comprehensions! As you hint at in the linked issue, I think this might better align with a tool with broader scope pyflakes, which autoflake focuses on.