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
PERF #6570 patch by @jreback
Merge pull request #6570 from hayd/groupby_selected_obj

FIX use selected_obj rather the obj throughout groupby
Merge pull request #6406 from filmor/patch-2

Make to_csv return a string if no path or buffer is provided.
ENH: including offset/freq in Timestamp repr (#4553)