TST/BUG: fix 2to3 import rewrite of import pickle

In pandas.io.pickle the following statement will not allow python to import
anything due to the following rewrite by 2to3

import pickle -> from . import pickle

This make the import try to import itself ad infinitum and thus fails.
TST: make tox block when running in parallel
Merge pull request #4063 from cpcloud/tox-pkl-import-fix

TST/BUG: fix 2to3 import rewrite of import pickle
CLN: clean up data.py
Merge pull request #4002 from cpcloud/data-dot-py-cleanup

CLN: clean up data.py
ENH: Add ``layout`` keyword to DataFrame.hist() for customizable format

GH4050
Add release notes for new ``layout`` keyword - GH4050
Merge pull request #4031 from danbirken/add_layout_to_frame_histogram

Add layout keyword to dataframe.hist()
TST: more data.py cleanup
Merge pull request #4085 from cpcloud/data-dot-py-cleanup

ENH: really really really fix the failing data.py tests
BUG: pass sharex and sharey in grouped_hist
Merge pull request #4091 from cpcloud/fix-share-params-grouped-hist

BUG: pass sharex and sharey in grouped_hist
fixed conflict in sql.py
BUG: GH4098, HDFStore not recreating a datetime index properly when has a timezone
Merge pull request #4099 from jreback/hdf_tz

BUG: GH4098, HDFStore not recreating a datetime index properly when has a timezone
BUG: (GH 4096) block ordering is somewhat non-deterministic in HDFStore, so reorder to the existing store

     if needed
Merge pull request #4100 from jreback/hdf_buglet

BUG: (GH 4096) block ordering is somewhat non-deterministic in HDFStore; reorder to the existing store
CLN: Refactor string methods and add PandasObject

Previous PandasObject becomes PandasContainer. New PandasObject becomes
baseclass for more elements (like Index, Categorical, etc.), moves
string methods to baseclass and subclassing objects need only define
`__unicode__` methods to get all string methods for free (and Py2/3
compatible).

CLN: Cleanup extraneous str methods from Panel

CLN: Remove unnecessary string methods from frame

CLN: Change name of TestPandasObjects --> TestPandasContainer
CLN: Make more core objects inherit PandasObject
CLN: Make Categorical inherit from PandasObject

CLN: Make GroupBy inherit from PandasObject

CLN/ENH: Make Sparse* into PandasObjects

Plus get all the string methods working...

CLN: Index now a PandasObject + str method cleanup

CLN: Make tseries/index fit with PandasObject.

CLN: Use PandasObject in internals + cleanup

CLN: Make Period into a PandasObject + cleanup

CLN: Remove extraneous __repr__ from io/excel
CLN: Have PyTables, stats, & Stata use StringMixin

CLN: Make PyTables unicode safe + add StringMixin

CLN: Make StataMissingValue use StringMixin

ENH: Use StringMixin for addl string methods in stats