CLN: Replace comprehensions list/set/dict functions with corresponding symbols
- [x] tests passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

xref @jreback's [comment](https://github.com/pandas-dev/pandas/pull/18335#discussion_r151851542) and @maxim-lian [comment](https://github.com/pandas-dev/pandas/pull/18335#discussion_r151843012), replace the following where appropriate:

`list(x for x in iterator)` --> `[x for x in iterator]`
`set(x for x in iterator)` --> `{x for x in iterator}`
`dict((x,x) for x in iterator)` --> `{x: x for x in iterator}`