Transition duration incorrect in CMX 3600

Hey, guys,

The [cmx_3600.py](https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/master/src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py) adapter appears to use a duration for Transitions that doesn't conform to the CMX 3600 spec.

According to [this spec](https://xmil.biz/EDL-X/CMX3600.pdf) (Page 30), the duration is stored in a "RATE" field.

[This guide](http://www.edlmax.com/EdlMaxHelp/Edl/maxguide.html) makes it a little easier to read:
```
75 frame dissolve from reel 123 to reel 123B on video and A1:

Edit # Reel Name Channel Trans Dur Source IN Source OUT Record IN Record OUT
105 123 B C 03:05:57:17 03:05:57:17 01:00:21:20 01:00:21:20
105 123B B D 075 03:15:33:09 03:15:35:24 01:00:21:20 01:00:24:05
```

However, the `cmx_3600.py` adapter appears to use the length of the previous clip:
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/84c8245c070b12d7e627c145435a5db9add0d2de/src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py#L668

Test EDL:
[cmx_3600_transition.edl.zip](https://github.com/PixarAnimationStudios/OpenTimelineIO/files/6039542/cmx_3600_transition.edl.zip)

Recreation Steps:
```
In [13]: cat cmx_3600_transition.edl
TITLE: 0800_Cut_v3_ToLayout_021121_EDL
FCM: NON-DROP FRAME
001 ABC0100. V C 01:00:12:15 01:00:17:01 01:00:48:20 01:00:53:06
002 ABC0200. V C 01:00:11:07 01:00:14:09 01:00:53:06 01:00:56:08
003 ABC0200. V C 01:00:14:09 01:00:14:09 01:00:56:08 01:00:56:08
003 ABC0300. V D 026 01:00:10:08 01:00:13:09 01:00:56:08 01:00:59:09
* BLEND, DISSOLVE
004 ABC0400. V C 01:00:11:09 01:00:17:20 01:00:59:09 01:01:05:20

In [14]: import opentimelineio as otio

In [15]: timeline = otio.adapters.read_from_file("cmx_3600_transition.edl")

In [16]: timeline.tracks[0][2].duration()
Out[16]: otio.opentime.RationalTime(value=73, rate=24)
```

Start in src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py around the transition-duration logic at line 668. Read the CMX 3600 specification's RATE field and run the attached cmx_3600_transition.edl reproduction through opentimelineio.adapters.read_from_file. Done means the parsed transition duration follows the specified RATE value rather than the previous clip's length.

Written by the indexing model from the issue text.