Add deprecated property to DefFrame for CHANNEL-DELAY attribute

[mac01021]

Description

v4 has added deprecated attribute properties for all the standard defframe attributes, except for CHANNEL-DELAY. I have a bunch of call sites in my v3 codebase that use this attribute and engineers like to benefit from autocompletion in their IDE when accessing channel delays. So here is a PR that adds the missing property.

Checklist

• [x] The PR targets the master branch
• [x] The above description motivates these changes.
• [x] The change is atomic and can be described by a single commit (your PR will be squashed on merge).
• [x] All changes to code are covered via unit tests.
• [x] Parameters and return values have type hints with PEP 484 syntax.
• [x] Functions and classes have useful Sphinx-style docstrings.
• [ ] (New Feature) The docs have been updated accordingly.
• [ ] (Bugfix) The associated issue is referenced above using auto-close keywords.

[mac01021]

@MarquessV Given that I've written this, are you willing to merge it? Or do you view it as a step in the wrong directions?