PERF: fix slow s.loc[[0]]

Fixes GH9126
fixup unicode compat with 3.2 in test_index.py
Merge pull request #9135 from jreback/fix_32

fixup unicode compat with 3.2 in test_index.py
Merge pull request #9127 from shoyer/fix-slow-loc

PERF: fix slow s.loc[[0]]
TST: Fix failing test due to revision in WB data #9115
BUG: fix isnull behavior when passed PeriodIndex with NaT (GH 9129)
PERF: use unique and isnull in nunique instead of value_counts.
Merge pull request #9137 from jnmclarty/fixwbtests

TST: Fix failing test due to revision in WB data #9115
Merge pull request #9134 from unutbu/nunique-unique

PERF: use unique and isnull in nunique instead of value_counts.
use labels to find duplicates in multi-index (GH9125)
DOC: minor updates in faq