ENH: Add unique and nunique to GroupBy whitelist

Series.unique and Series.nunique are useful methods for querying the
unique elements (or number of unique elements) in each group when used
in conjunction with DataFrame.GroupBy.  This commit addresses
pydata/pandas#6146.
ENH: Updated tests for series GroupBy whitelist

Added tests for fix to pydata/pandas#6146.
Merge pull request #6147 from bburan-galenea/bburan/groupby-series-unique

ENH: Add unique and nunique to GroupBy whitelist
CLN: fix ScatterCI links in README and rnotes
CLN: convert test docstring to comment
BLD/TST: catch more spurious errors in @network decorator
Merge pull request #6151 from y-p/PR_network

BLD/TST: catch more spurious errors in @network decorator
BLD/TST: one more skip_errno for @network
BUG: fix loc setitem with a dataframe on rhs, multiple items, and a datetimelike (GH 6152)
Merge pull request #6152 from jreback/loc

BUG: fix loc setitem with a dataframe on rhs, multiple items, and a datetimelike
BLD/TEST: @network, SkipTest on known errnos
PERF: perf regression index construction from seris (GH6150)
Merge pull request #6153 from jreback/index_perf

PERF: perf regression index construction from series (GH6150)
TST: rec arrays don't support datetimes in creation on certain platforms, related (GH6140)
TST: dtype comparison issue on windows for (GH6152)
Merge pull request #6156 from jreback/sparc_fix4

TST: rec arrays don't support datetimes in creation on certain platforms, related (GH6140)
Updated docs to reflect a pagination bug that was fixed. Closes: issue #6096
Merge pull request #6157 from jacobschaer/master

Updated docs to reflect a pagination bug that was fixed. Closes: issue #6068
BUG: allow lex string comparisons
Merge pull request #6158 from cpcloud/eval-fix-str-lexico

BUG: allow lex string comparisons