BUG: GH3416 properly convert np.datetime64 objects in the _possibily_convert_datetimes
CLN: _possibily_convert_datetimes now uses to_datetimes internally
BUG: fixes GH3425, raising on passed invalid dtypes for datetimelike

     fixes GH2423, astyping is now checked when using datetimelike and timedeltalike
     for valid astype to dtypes

BUG: PY3 compat for timedelta64[ns] in astype

BUG: more py3 compat

PTF
Merge pull request #3516 from jreback/GH3416

BUG/CLN: datetime64/timedelta64