DOC: Clarify that DataFrame string formatters must return strings

I am not 100% sure that string formatters must really return *unicode* strings
(non-unicode strings seem to work, too), but I am leaving that because it's
what is already documentated.

Fixes #6052.
ENH: allow legend='reverse' in df.plot() GH6014
ENH: Import testing into main namespace.
TST: run clipboard tests explicitly
Update indexing.rst

Just removing a typo: 'for' was written twice.
Merge pull request #6255 from michaelaye/patch-1

Update indexing.rst
Merge pull request #6256 from jreback/loc_bug

BUG: Indexing bugs with reordered indexes (GH6252, GH6254)
TST: dtype fixes for #6256
BUG: bug in xs for a Series with a multiindex (GH6258)
TST: extra test for GH5684
CLN: More work on converting assert_'s to specialized forms

Work on #6175. Tests still pass locally. A work-in-progress.
Merge pull request #6259 from jreback/xs_bug

BUG: bug in xs for a Series with a multiindex (GH6258)
CLN: Work on converting assert_'s to specialized forms, in pandas/tests

Work on #6175, in pandas/tests. Tests still pass locally. A work-in-progress, but--with #6261--removes many/most cases of assert_(x == y) in this directory
Merge pull request #6261 from bwignall/assert_6175_4

CLN: More work on converting assert_'s to specialized forms
Merge pull request #6262 from bwignall/assert_6175_5

CLN: Work on converting assert_'s to specialized forms, in pandas/tests
DOC: mention display.mpl_style in viz.rst
BUG: Bug in conversion of a string types to a DatetimeIndex with a specified frequency (GH6273)
TST: add tests for GH6274
Merge pull request #6275 from jreback/series_to_index

BUG: Bug in conversion of a string types to a DatetimeIndex with a specifed frequency (GH6273)
DOC: fix link to str.get_dummies