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* improved exceptions for time differences (issue 128, PR 131).
* fix intersphinx entries (issue 133, PR 133)
* make only_use_cftime_datetimes=True by default, so cftime datetime
instances are returned by default by num2date (instead of returning python
datetime instances where possible). Issue 136, PR 135.
* Add daysinmonth attribute (issue 137, PR 138).
* If only_use_python_datetimes=True and only_use_cftime_datetimes=False,
num2date only returns python datetime instances and raises an exception
if this is not possible. num2pydate convenience function added which just calls
num2date with only_use_python_datetimes=True and
only_use_cftime_datetimes=False.
Remove positive times check, raise ValueError if python datetime
tries to compute a date before MINYEAR (issue 134, PR 139)
* Fix for fractional seconds in reference date in units string (issue 140,
PR 141).