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
CLN: Converting assert_'s to specialized forms, in pandas/*/tests

Work on #6175, in pandas/*/tests. Tests still pass locally. Finishes most (all?) of the remaining vanilla assert_(x == y) conversions
Merge pull request #6118 from y-p/PR_reverse_legend

ENH: allow legend='reverse' in df.plot() GH6014
DOC: release notes
Merge pull request #6267 from bwignall/assert_6175_6

CLN: Converting assert_'s to specialized forms, in pandas/*/tests
DOC: clean-up docstrings of sql
BLD: caching hanging when numstat is long
BLD: fix hanging build cache again
DOC: fix sql docs (write example data to database)
Update groupby.rst
BLD: disable MySQL in requirements-2.7 as its failing the building process
API: allow the iloc indexer to run off the end and not raise IndexError (GH6296)
Merge pull request #6299 from jreback/iloc_max

API: allow the iloc indexer to run off the end and not raise IndexError (GH6296)
clarified to_excel and read_excel

as discussed in issue GH6260
. io.rst: added a paragraph explaining the difference in behaviour from
0.12
. excel.py: added docstring for has_index_names
thanks to jmcnamara and jorisvandenbossche
Update groupby.rst

Edit following this remark <https://github.com/pydata/pandas/pull/6304/files#r9564092>
CLN: Convert last of assert_(x == y) to assertEqual

Work on #6175. Fixes last of vanilla assert_(x == y) conversions in pandas/tests and pandas/*/tests. Tests still pass locally.
CLN: Convert assert_(x != y) to assertNotEqual

Work on #6175. Converts instances in pandas/tests and pandas/*/tests. Tests still pass locally.