FIX raise when groupby selecting cols not in frame
DOC: whatsnew edits
Speed up DatetimeConverter by using Matplotlib's epoch2num when possible.
TST: skip sql tests if connection to server fails
Merge pull request #6651 from jorisvandenbossche/sql-skiptests

TST: skip sql tests if connection to server fails
Added vbench performance benchmark for speed up of DatetimeConverter.
SQL: move df.to_sql to generic + update with new functionality
Merge pull request #6598 from jorisvandenbossche/sql-to_sql

SQL: move df.to_sql to generic + update with new functionality
BUG: disallow mixed dtype operations
DOC: some minor doc fixes
Merge pull request #6653 from jorisvandenbossche/doc-fixes2

DOC: some minor doc fixes
PLAT: platform sorting issue surfaced with time_grouper
Merge pull request #6649 from pydata/platform_sort

PLAT: platform sorting issue surfaced with time_grouper
DOC: update groupby pd.Grouper docs
CLN: move set_grouper/get_grouper type pd.Grouper methods to internal
Merge pull request #6655 from jreback/groupby

DOC/API: pd.Grouper docs / api
Merge pull request #6652 from cpcloud/gh-5198-stack-blow-eval-mixed-type-operations

BUG: disallow mixed dtype operations in eval/query
Fixes to DatetimeConverter speed up following PR comments and corresponding note in release notes..
Fix failed tests when importing Series from pandas.
Merge pull request #6650 from agijsberts/speed-up-dateconverter

PERF: Speed up DatetimeConverter by using Matplotlib's epoch2num when possible...