PEP 808: backwards compatibility example update

[henryiii]

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This adds a backward compat example for static tooling based on @sirosen's comment in https://discuss.python.org/t/104883. This shows an example of how to read this in Python, along with a note about how it could have been done before in a way that would be incorrect now (only in that you wouldn't know the metadata you were computing was incomplete).

Tested which the following quickly thrown together snippet:

class my:
    key = "dependencies"


def f(pyproject):
    match pyproject["project"]:
       # New in PEP 808
       case {my.key: value, "dynamic": dyn} if my.key in dyn:
           print(f"Partial {my.key}: {value}")
       case {"dynamic": dyn} if my.key in dyn:
           print(f"Fully dynamic {my.key}")
       case {my.key: value}:
           print(f"Fully static {my.key}: {value}")
       case _:
           print(f"No metadata for {my.key}")


f({"project": {"dynamic": ["dependencies"], "dependencies": ["numpy"]}})
f({"project": {"dynamic": ["dependencies"]}})
f({"project": {"dependencies": ["numpy"]}})
f({"project": {}})

(Marked as draft until @sirosen (and maybe @pfmoore) can verify this addresses their comments)

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