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).
ENH: Add functions for creating dataframes with NaNs
DOC: add explanation to doc/sphinxext
Merge pull request #6442 from jorisvandenbossche/doc-fixes

DOC: small doc fixes
Merge pull request #6395 from bwignall/assert_6175_innotin

CLN: Change assert_(a [not] in b) to specialized forms
TST: fix spurious tests for test_groupby (GH6436)
BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers to promotoe
         consistency with python/numpy syntax. The out-of-bounds for slice indexers
         will continue to work (again for consistency) (GH6296 / GH6299)
DOC: fix .tz attribute error in DatetimeIndex when building docs
Merge pull request #6443 from jreback/iloc_bounds2

BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers (GH6296 / GH6299)
CLN: minimize tokenizer passes
Merge pull request #6432 from cpcloud/eval-consolidate-tokenization

CLN: minimize tokenizer passes
INT/CLN: clean up block slicing semantics
Merge pull request #6446 from jreback/iloc_tests

INT/CLN: clean up block slicing semantics
str_extract should work for timeseries, bug 6348
BUG FIX: cartesian_product now converts all arguments to ndarrays

Fixes GitHub issue #6439.
BUG/TST: sorting of NaNs on sym_diff
CLN: remove dead code in pandas.computation
Merge pull request #6448 from cpcloud/eval-coverage

CLN: remove dead code in pandas.computation
Merge pull request #6453 from TomAugspurger/sym_diff_nan3.4

BUG/TST: sorting of NaNs on sym_diff
Merge branch 'fix-cartesian-product' of https://github.com/shoyer/pandas into shoyer-fix-cartesian-product