DOC Improve OPTICS doc (scikit-learn#13866)

DOC Fix reference in whats new 0.21 (scikit-learn#13845)

BLD retagging as 0.21rc2

Version = 0.21rc1

TST use approximate equality for float comparison (scikit-learn#13749)

DOC release dates for 0.20.3

DOC update version to 0.20.2

don't set check_pickle on new joblib on python2.7 (scikit-learn#12645)

ColumnTransformer generalization to work on empty lists (scikit-learn…

…#12084)

MNT rc version