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
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).
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
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)