Only depend on setuptools-git-ls-files for sdist

[tjni]

Some PEP 517 build frontends, like PyPA build, have a feature to check that build dependencies are present prior to starting the build.

The setuptools-git-ls-files dependency is only needed when constructing the sdist but not when building a wheel. Some package managers, such as nixpkgs, only build wheels but still need to bundle and depend on this to pass validation.

This commit adds some complexity to specify that setuptools-git-ls-files is only needed when building an sdist by extending setuptools.

This is the same solution implemented in https://github.com/adobe-type-tools/cffsubr/pull/23.

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[tjni]

Oh, I'm sorry, I didn't notice that this requires signing the Google CLA. I'm not interested in reading and signing it. Please feel free to do whatever you'd like with the code, however.

[anthrotype]

@behdad I'm not against the changes but the author stated they do not wish to sign the CLA so am not sure how to proceed here. Also there's new CI build failures which I think are related to latest cibuildwheel attempting to build wheels for Python 3.12 (now release candidate so ABI stable) but compreffor somehow failing for that (we may need to regenerate the cython .cpp files with a more up-to-date cython to support 3.12).

[davelab6]

I guess we will reject this and the next nix user who will agree to the CLA will make a similar PR that we are allowed to merge.