cmx_3600 adapter incorrectly processing three-clip transitions

I have a case of a three part Transition in a CMX 3600 EDL. It looks like so:

Clip A -> Dissolve -> Clip B -> Dissolve -> Clip C

Here's the EDL:
```
TITLE: TRANSITION_TEST_2
FCM: NON-DROP FRAME
001 ABC0000. V C 01:00:06:18 01:00:08:00 01:04:11:17 01:04:12:23
002 ABC0010. V C 01:00:06:15 01:00:08:18 01:04:12:23 01:04:15:02
003 ABC0010. V C 01:00:08:18 01:00:08:18 01:04:15:02 01:04:15:02
003 ABC0020. V D 035 01:00:06:22 01:00:10:07 01:04:15:02 01:04:18:11
* BLEND, DISSOLVE
004 ABC0020. V C 01:00:10:07 01:00:10:07 01:04:18:11 01:04:18:11
004 ABC0030. V D 064 01:00:06:10 01:00:09:22 01:04:18:11 01:04:21:23
* BLEND, DISSOLVE
005 ABC0040. V C 01:00:08:14 01:00:12:14 01:04:21:23 01:04:25:23
```

In that example, ABC0020 should have a duration of 96, however OTIO it showing it to have 50 frames.

There appears to be something specifically wrong here, in the logic that derives what is the "previous clip":
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/84af5e5a30e4d14cf3434d6d055631e115ac1ee5/src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py#L706-L717

Reproduce the supplied three-clip CMX 3600 EDL and inspect src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py around lines 706-717, where the previous clip is derived. Compare the generated duration for ABC0020 with the expected 96 frames; done means the adapter reports 96 rather than 50 for this transition.

Written by the indexing model from the issue text.