gh-113317: Add ParseArgsCodeGen class

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• Issue: Argument Clinic: split out global stateless helpers and constants from clinic.py #113317

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This PR is based on PR gh-117626.

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PR rebased on top of merged #117626.

[erlend-aasland]

The previous PR used the Codegen spelling. This PR uses CodeGen. Consider using only one variant :)

[erlend-aasland]

This is nice!

Should we consider adding this as a separate module? Or would libclinic.codegen be a better home for it?

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Should we consider adding this as a separate module?

CLanguage uses ParseArgsCodeGen which uses CLanguage... There is an inter-dependency. It's more convenient to have both classes in the same file.

[erlend-aasland]

Yes, there are inter-dependencies everywhere :(

[vstinner]

The previous PR used the Codegen spelling. This PR uses CodeGen. Consider using only one variant :)

I updated this PR to rename Codegen to CodeGen.

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Should we consider adding this as a separate module?

I broke the inter-dependency between CLanguage and ParseArgsCodeGen, and I moved declare_parser() and ParseArgsCodeGen to a new libclinic.parse_args module.

[erlend-aasland]

I broke the inter-dependency between CLanguage and ParseArgsCodeGen, and I moved declare_parser() and ParseArgsCodeGen to a new libclinic.parse_args module.

Nice, so now we can make the parser templates globals of the libclinic.parse_args module, instead of class members.

[vstinner]

Nice, so now we can make the parser templates globals of the libclinic.parse_args module, instead of class members.

Done.