move PyDict::get_item_with_error to PyDict::get_item

[davidhewitt]

As per capi-workgroup/problems#51 the recommendation of the Python core devs is to avoid PyDict_GetItem because it can hide arbitrary exceptions.

Accordingly I've moved the PyDict::get_item_with_error to just become PyDict::get_item, and deprecated the get_item_with_error name.

[davidhewitt]

This is obviously quite an irritating change for users because it changes the return type from Option<&PyAny> to PyResult<Option<&PyAny>>. This means adding an extra .unwrap() in a lot of our test code. For users it's probably less bad because they can hopefully add ? to propagate the exception.

I still defend the change as worthwhile given it nudges them onto using a "correct" API instead of unintentionally ignoring inner errors.

[mejrs]

+1 for this change.

It'd be nice to flesh out the docs. e.g. to answer questions like

• is it OK to unwrap or unwrap_or(None)?
• should I just bubble up the error?
• when can this return an error?

[davidhewitt]

Sorry for the long delay here. I've added some detail to the documentation to cover the points raised above, they were all good suggestions 👍

[davidhewitt]

As this has had an approval and I think any further changes which might be requested from further reviews would be docs rather than API, I'm going to merge this now so that I can start building an 0.20 release branch.