When task has start and stop that are same dates, gantt can't be constructed

[golharam]

In gantt.py, line 886, if a task was created with a start and stop date being the same date (and not with duration), the while loop incorrectly reference duration without it being set.

The error is:
TypeError: '>' not supported between instances of 'NoneType' and 'int'

Python-Gantt/gantt/gantt.py

Line 876 in 0723709

while duration > 1 or (current_day.weekday() in _not_worked_days() or current_day in VACATIONS):

[golharam]

Addressed by #3