CPython engine marshals python classes to lists when the expectation is to get a handle.

[mjkkirschner]

Purpose

While trying to get data-shapes working with the cpython engine it was discovered that some of the data-shapes python classes implement __iter__/__getitem__ and sequence protocols.

These classes are marshaled into dynamo as lists, but for use in data-shapes they need to be marshaled as handles only.

To enable this workflow I've added a new "builtin" 👋 attribute the class author can choose to add to their class. __dynamoskipconversion__. If this attribute exists at all (the return value does not matter) then the object will skip all conversion attempts and immediately be marshaled as a handle.

For dataShapes this means a one line code change to get this class working.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• When/If this PR is approved I will update the python3 wiki with information on this attribute.

[mmisol]

Looks good. I left some minor comments