BUG: Correct importing behavior for Categoricals in StataReader

Ensure that category codes have the same order as the underlying Stata data.
Also adds a flag that allows categorical data to be treated as ordered or
unordered when importing.
BUG: fix negative step support for label-based slices

INT: make Index.slice_locs step-aware

BUG: fix PeriodIndex.searchsorted to accept Periods

INT: refactor time-related indices to use step-aware slice_locs

INT: refactor MultiIndex to use step-aware slice_locs

INT: enable second/microsecond partial string slicing
Merge pull request #8836 from bashtage/stata-monotonic-categoricals

FIX: Correct Categorical behavior in StataReader
Clarify encoding kwarg on to_csv
DOC: Clean up Stata documentation

Clean up and simplification of Stata documentation.

[skip ci]
Merge pull request #8753 from immerrr/refactor-slice-locs

API: allow negative steps for label-based indexing
BUG: DataFrame.stack(..., dropna=False) with partial MultiIndex.
Merge pull request #8858 from bashtage/stata-doc-fixup

DOC: Clean up Stata documentation
Merge pull request #8857 from graingert/patch-1

Clarify encoding kwarg on to_csv
BUG: type change breaks BlockManager integrity
Merge pull request #8853 from behzadnouri/blk-mgr

BUG: type change breaks BlockManager integrity
index into multi-index past the lexsort depth
BUG: missing nose import for skip test

nose.SkipTest used but nose not imported.

Import nose at top level and use that throughout.
Merge pull request #8867 from matthew-brett/missing-nose-import

BUG: missing nose import for skip test
Merge branch 'mi-key-loc' of https://github.com/behzadnouri/pandas into behzadnouri-mi-key-loc
TST: provide a clear argument to assert_produces_warning to guarantee that we are clearing
     specific warnings before trying to catch them (in case they have already happened)
     requires the user to specificy the exact class where they are coming
DOC: Add where and mask to API doc
DOC: Add Nano to offsets table
Merge pull request #8879 from sinhrks/nanodoc_2

DOC: Add Nano to offsets table
Merge pull request #8876 from sinhrks/doc_where

DOC: Add where and mask to API doc