STY: proposed isort settings [ci skip] [skip ci] [ciskip] [skipci]
(not sure which tag(s) skip the CI)

I propose we change the isort settings to arrange within-pandas imports by dependency structure.  i.e. 

- `pandas._libs`, `pandas.compat`, `pandas.util._*`, `pandas.errors` are for the most part not dependent on pandas.core, so get their own section(s)
- `pandas.core.dtypes` for the most part does not depend on the rest of `pandas.core`, so it goes next
- then the rest of `pandas.core`
- then pandas non-core: `pandas.io`, `pandas.plotting`, `pandas.tseries`

Within blocks I propose we stick to alphabetical ordering.  e.g. right now in `core.series` we have import `pandas.core.indexes.base as ibase` 35 lines away from all the other `pandas.core.indexes` imports.  These should be adjacent.

The main thing I'd still like to fix/improve is that in this PR isort is failing to put `from pandas import compat` with the other `pandas.compat` imports.

Putting the trailing parentheses on the same line as the imports instead of a separate line is less important, but I prefer it because it makes it cleaner when I wrap lines in sublimetext.  Happy to revert that if it is a sticking point.