CLN: PEP8 cleanup

I stated a PEP8 cleanup and did it for a few modules. Changed only the things where this doesn't produce ugly code.

If this is acceptable, I can continue doing this and go through the whole pandas codebase. There are just a few things that I would like to check if this is acceptable:

1. PEP8: "For flowing long blocks of text with fewer structural restrictions (docstrings or comments), the line length should be limited to 72 characters." - should I do this?
2. Should I clean the unused imports? There are a lot of unused imports, not just from pandas but also from the standard library. Also duplicate imports (importing `timedelta` 2 times for example in the cleanup that I did)...
3. Should I discard the unused variables? (There are a lot of them especially in the tests).
4. `a[i+2:3]` this is against PEP8, it should look like `a[i + 2:3]`, but I don't think these things should be changed, the second one is uglier even if it's against PEP8.