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: Import testing into main namespace.
TST: run clipboard tests explicitly
Update groupby.rst
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.
Merge pull request #6308 from bwignall/assert_6175_7

CLN: Convert last of assert_(x == y) to assertEqual
BUG: Regression in join of non_unique_indexes (GH6329)
Merge pull request #6330 from jreback/align

BUG: Regression in join of non_unique_indexes (GH6329)
BUG: Issue with groupby agg with a single function and a a mixed-type frame (GH6337)
Merge pull request #6338 from jreback/agg

BUG: Issue with groupby agg with a single function and a a mixed-type frame (GH6337)
BLD: remove -v from nosetests for shorter output
BUG: fix bool no-op replace calls
CLN: replace in bool block rather than special-casing in common
TST: add tests for dict with bool keys
DOC: release notes
TEST: add basic postgresql tests

TEST: add postgresql to travis
TEST io.sql: sqlite tests to seperate class

One base class for tests with sqlalchemy backend. So test classes
for mysql and postgresql don't have to overwrite tests that are
different for sqlite.
Merge pull request #6339 from cpcloud/replace-bool-fix

BUG: fix bool no-op replace calls