workaround a dateutil bug

In the latest released version of dateutil, there's a bug whereby a TypeError
can be raised whilst parsing a date. This is because it calls a method which
it expects to return 2 arguments, however it can return 1 depending upon the
input, which results in a TypeError: ArgType not iterable exception. Since
this is equivalent to a failed parse anyway, we can treat it the same as a
ValueError.