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]]
BUG: fix isnull behavior when passed PeriodIndex with NaT (GH 9129)
PERF: use unique and isnull in nunique instead of value_counts.