TST: Mark datetime test as a known failure on Python's below 2.7.
TST: Mark datetime test as a known failure on Python's below 2.7.

Appears that there is an attribute used by this test, `total_seconds` of
`timedelta` objects that is not introduced until Python 2.7. So, this
does not exist on Python 2.6. Here we mark this test as a known failure
on Python's below 2.7.

ESSS/numpy,ESSS/numpy,ESSS/numpy