Task attribute "entry" should be editable

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](https://taskwarrior.org/docs/terminology.html#end) 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.
```