ENH: Import testing into main namespace.
CLN: Specialize assert_(np.array_equal(...))

Work on #6175. This is the third (and final) "half" of the work started in #6368.
Fix behavior of `to_offset` with leading zeroes

Currently, `pandas.tseries.frequencies.to_offset` erroneously returns a zero time offset when the first part of its argument has a numerical value of zero, even if later parts have nonzero values.  For instance,

    In [123]: pandas.tseries.frequencies.to_offset('00H 00T 01S')
    Out[123]: <0 * Days>

In this patch the sign check is applied before conversion to `int` in order to support offsets like `'-00H 00T 01S'`.
API/CLN: add in common operations to Series/Index, refactored as a OpsMixin
         (GH4551, GH4056, GH5519)
TST: Add tests for `to_offset` with leading zeroes

Test for the issues fixed by cbc8c73fdf617a4644baa130dff05476c7a61645.
Merge pull request #6380 from jreback/common_ops

API/CLN: add in common operations to Series/Index, refactored as a OpsMixin
Merge pull request #6387 from bwignall/assert_6175_arrayequal_3

CLN: Specialize assert_(np.array_equal(...))
CLN: Change assert_(a is [not] b) to specialized forms

Work on #6175. Changes instances of assert_(a is [not] b) to specialized assertIs[Not](a, b).
Merge pull request #6392 from bwignall/assert_6175_isisnot

CLN: Change assert_(a is [not] b) to specialized forms
BUG: Regression in chained getitem indexing with embedded list-like from 0.12 (6394)
DOC: release.rst update
Merge pull request #6396 from jreback/index_regr

BUG: Regression in chained getitem indexing with embedded list-like from 0.12 (GH6394)
Add `to_offset` fix to release notes
TST: dtype issues on windows with test_getitem_dups
TST: disable odd test_data/test_fred tests failing, maybe a data revision?
DOC: missing spaces in release.rst
DOC: more release.rst fixes
Merge pull request #6391 from ischwabacher/patch-1

BUG: Fix behavior of to_offset with leading zeroes
Merge pull request #6188 from jseabold/import-testing

ENH: Import testing into main namespace.
BUG: Fix for GH6399, mergesort is unstable when ascending=False

BUG: Fix for GH6399, mergesort is unstable when ascending=False. Update changes