ENH: TimedeltaIndex and Timedelta scalar support (GH3009)

BUG: coercion of timedeltas in assignment (GH8209)

TST: skip sql.timedelta tests for now

COMPAT: provide conversion for passing in non-ns datetime64/timedelta64

BUG: diff on Dataframe with timedelta64 broken (GH4533)
DOC: move timedeltas to new section
ENH: support for alternate Timedelta format constructors in natural language (GH8190)
fixup astyping / freq conversion