Incorrect warning in `detect_scenes` when `end_time` is not a `FrameTimecode`

[Breakthrough]

Description:

As @Vsevosemnog pointed out in #345, a runtime warning is emitted when passing strings for end_time to SceneManager.detect_scenes:

[2023-08-10 09:00:49,774] INFO - '<' not supported between instances of 'str' and 'int'

This is caused by the comparison on this line. The timecode is converted after the comparison below.

The code should still function even with the incorrect comparison, but we should avoid doing this check if the input is a string.