python: Implement version-agnostic Protocol types

[bact]

This PR adds src/shacl2code/lang/templates/python/protocols.py.j2 template to generates Protocol structure types for classes in the generated bindings.

Given two versions of a model (v1 and v2; v2 is backward compatible with v1) to shacl2code to generate Python bindings (one at a time), it will generate two bindings that look similar but v1.SomeClass and v2.SomeClass are considered distinct. They are not sharing a common parent type.

This makes it difficult for a downstream user who wants to write a function like:

def func(x: SomeClass) -> SomeClass: ...

Because if it defines SomeClass as v1.SomeClass, it will be incompatible with v2.SomeClass (and vice versa).

This PR provides the generation of protocols.SomeClass which is a structural type that accepts any class with the same shape (has similar members), so v1.SomeClass and v2.SomeClass can be both accepted, even they are unrelated.

This function will work across all versions of the model (provided that they are keeping backward compatibility to previous versions -- not removing any members in newer versions):

def func(x: protocols.SomeClass) -> protocols.SomeClass: ...

[github-actions]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/shacl2code/lang
python.py
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