BUG: Timestamp - Timestamp should be pd.Timedelta, not datetime.timedelta

[shoyer]

Currently it returns a datetime.timedelta object:

>>> pd.Timestamp('2000-01-01') - pd.Timestamp('1999-01-01')
datetime.timedelta(365)

Somewhat related: #8813

[jreback]

hmm thought this was tested
should for sure return a Timedelta