BUG: Series.quantile raising on an object dtype (GH6555)
ENH: Allow timestamp and data label to be set when exporting to Stata

Added code which allows the time stamp and the data label to be set using
either StataWriter or to_stata.  Also simplified reading these values using
StataReader by removing null bytes from the string values read.

Added basic test for both.

Also fixed one small bug where variables could be stored using Stata reserved
words.
BUG: preserve frequency across Timestamp addition/subtraction (#4547)
Merge pull request #6558 from jreback/quantile_object

BUG: Series.quantile raising on an object dtype (GH6555)
Merge pull request #6553 from bashtage/time_stamp-data_label-reserved_words

ENH: Allow timestamp and data label to be set when exporting to Stata
Merge pull request #6560 from rosnfeld/issue_4547

BUG: preserve frequency across Timestamp addition/subtraction (#4547)
ENH/BUG groupby nth now filters, works with DataFrames
TST add vbench for groupby nth
Merge pull request #6569 from hayd/groupby_nth

ENH/BUG groupby nth now filters, works with DataFrames
BLD: fix versions of setuptools/pip/wheel to be stable and know to work
DOC: cookbook.rst entry
CLN: Change assert_([not] isinstance(a,b)) to specialized forms

Work on #6175. Changes instances of assert_([not] isinstance(a,b)) to specialized assert[Not]IsInstance(a, b).
CLN: Change assert_([not] isinstance(a,b)) to specialized forms

Work on #6175. Change superclass of tests/test_compat, to use tm.TestCase
BUG: Bug in .xs with a nan in level when dropped (GH6574)
Merge pull request #6577 from bwignall/assert_6175_isinstance

CLN: Change assert_([not] isinstance(a,b)) to specialized forms
Merge pull request #6579 from jreback/xs

BUG: Bug in .xs with a nan in level when dropped (GH6574)
CLN: Finish changing assert_(...) to specialized forms

Finishes #6175, to the extent that everything remaining can be mapped to assertTrue or assertFalse, unless larger refactorings are desired.
Merge pull request #6580 from bwignall/assert_6175_cleanup

CLN: Finish changing assert_(...) to specialized forms
Make to_csv return a string in case no buffer is supplied.

Fixes issue #6061.
FIX use selected_obj rather the obj throughout groupby

TST dont ignore subselection in groupby