ENH: Import testing into main namespace.
DOC: clarify docstring of rolling/expanding moments

- document center argument
- add note about the result set at right edge by default
- clarified freq keyword a little bit
DOC: fix doc build warnings
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'`.
TST: Add tests for `to_offset` with leading zeroes

Test for the issues fixed by cbc8c73fdf617a4644baa130dff05476c7a61645.
CLN: Change assert_(a [not] in b) to specialized forms

Work on #6175. Changes instances of assert_(a [not] in b) to specialized assert[Not]In(a, b).
Add `to_offset` fix to release notes
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
BUG: Float64Index with nans not comparing correctly
Merge pull request #6401 from jreback/float_equal

BUG: Float64Index with nans not comparing correctly
ENH: pd.infer_freq() will accept a Series as input
API: pd.infer_freq() will now raise a TypeError if given an invalid Series/Index type (GH6407)
Merge pull request #6408 from jreback/freq_ops

API: pd.infer_freq interacts correctly with a Series
API: validate conversions of datetimeindex with tz, and fixup to_series() to handle (GH6032)
Merge pull request #6398 from jreback/index_coerce

API: validate conversions of datetimeindex with tz, and fixup to_series() to handle (GH6032)
DOC: use ipython in bool replace doc warning
DOC: use code block for shorter error message
Merge pull request #6412 from cpcloud/replace-bool-doc-fix

DOC: use ipython in bool replace doc warning
TST: GH6410 / numpy 4328