implement PEP-563: annotations as strings

I think Cython 3.0 would also be a good time to implement PEP-563 (“Postponed Evaluation of Annotations”). There already is a Cython code generator in Cython.CodeWriter that can be used to generate the expression string. This should be done somewhat late in the compiler pipeline since some optimisations can make use of the annotation information. It might be worth creating a separate transformation before the code generation.

What needs to be done:

• Implement a new transform class SerialiseAnnotations(VisitorTransform) in ParseTreeTransforms.py.
• Give it visitor methods for all nodes that can have annotations (any ExprNode, and anything that has an annotation attribute in Nodes.py).
• If the node has an annotation, serialise it using the code writer and replace the existing node.annotation with a StringNode' (or UnicodeNode` ?) that wraps the string value.
• Add the new transform to the pipeline in Pipeline.py, rather late, before the final checks.
• Extend the existing annotation related tests in tests/run/ to cover the annotation string value. Add new tests for expressions that are not covered yet.

[scoder]

Any help is appreciated.

[scoder]

@nihilist-banana, there is a description of the implementation steps in the ticket, and the HackerGuide has some general information about how to get started. Does this help already, or otherwise, could you maybe ask on the cython-devel mailing list if you need more specific guidance?