Use a saner requirements for python-dateutil

The requirement ``>=1.0, <2.0, >=2.1`` doesn't make a lot of logical sense and it
will break in the future. There is no version that is ``>= 1.0``, and ``< 2.0``, and
``>= 2.1`` because these versions are mutually exclusive. Even if you interpret
the ``,`` as OR it still doesn't make sense because this includes every version.

What this spec is actually trying to represent is any version ``>= 1.0`` but not
``2.0``, so instead we'll just say that.