BUG/API: comparison datetime64-vs-dates

[jbrockmendel]

ser = pd.Series(["2016-01-01"], dtype="date32[pyarrow]")
ser2 = ser.astype("timestamp[ns][pyarrow]")
ser3 = ser.astype("datetime64[ns]")

assert (ser3 != ser[0]).all()
assert (ser3 != ser).all()

assert (ser == ser3).all()  # <- uh-oh! inconsistent with reversed operation
assert (ser == ser2).all()  # inconsistent with the non-pyarrow behavior
assert (ser2 == ser).all()

Long ago we made a decision that datetime64 arrays (and Timestamp) were not comparable to date objects, matching the stdlib behavior. We should have that behavior for pyarrow dtypes too.