small change to tests needed for upcoming cftime 1.3.0 release

[jswhit2]

PR #51 will ensure the tests pass with cftime >= 1.3.0.

cftime 1.3.0 will allow vanilla cftime.datetime instances to be calendar-specific using the calendar __init__ kwarg (the calendar specific sub-classes will no longer be required to instantiate 'calendar-aware' instances).

[jswhit]

cftime 1.3.0 has been released, and this apparently breaks nc-time-axis (see Unidata/cftime#210)

[bjlittle]

Closed by #66