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 becasue 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.