component._get_start_end_duration called but not defined on Component

[niccokunzmann]

Component._get_start_end_duration() is called here:

https://github.com/collective/icalendar/pull/896/files/b44b57b1cbbf9b1aed9535bb38035fa21de37358#diff-46392468ae661de0550d017d8667adaa696b9d0d32e5fa0ad746623844d2a315R1570

But it is not defined on the component.

To solve this issue:

• change the typing so that it is defined on all instances.

https://github.com/collective/icalendar/pull/896/files/b44b57b1cbbf9b1aed9535bb38035fa21de37358#diff-46392468ae661de0550d017d8667adaa696b9d0d32e5fa0ad746623844d2a315R1555

Originally posted by @niccokunzmann in #896 (comment)