Task attribute "entry" should be editable

[abone28]

When I try to assign a new value to entry attribute (task["entry"] = value) tasklib 1.3.0 raises error

File "tasklib/task.py", line 101, in __setitem__
    raise RuntimeError('Field \'%s\' is read-only' % key)
RuntimeError: Field 'entry' is read-only

According to TaskWarrior docs this attribute can be modified

When a task is first created, it is automatically assigned an entry date. This attribute is used to calculate the age of a task. This date is modifiable, should you wish to enter an old task.

TaskWarrior 2.5.1 CLI allows to modify entry attribute:

task 1 modify entry:20200518T180617Z
Modifying task 87 'task1'.
Modified 1 task.