Feat: Python version support for pycross_wheel_builder

[LaurenceTews]

Context

My team is looking to build wheels with multiple version of Python. We've been using the transition in this PR to add python_version support to the pycross_wheel_build rule. It uses the rules_python flag so that Bazel resolves the correct toolchain for the corresponding python_version. This only works for toolchains of type @rules_python//python:toolchain_type and not //pycross:toolchain_type.

Happy to add some tests and update the docs for this if it's something folks would be interested in.

[jvolkman]

This seems pretty useful. I put together a demo one time, but only setting flags via the CLI: https://github.com/jvolkman/bazel-pycross-zstandard-example

I wonder though whether there's a more composable way to accomplish this? Have you looked at something like https://github.com/fmeum/with_cfg.bzl? I think that should allow creating different version-specific pycross_wheel_build rules.

This only works for toolchains of type @rules_python//python:toolchain_type and not //pycross:toolchain_type

I guess this means this won't support cross-platform builds? What's preventing support for the pycross toolchain?

[LaurenceTews]

@jvolkman thanks for your response. with_cfg looks really interesting, it seems like a great option for modifying rules from external repos. In terms of using it inside of rules_pycross, it does create a slightly awkward invocation pattern, since it has to be called from a .bzl file. It would look something like this -

# defs.bzl
pycross_wheel_build_3_10 = versioned_pycross_wheel_build(python_version = "3.10.11")

# BUILD.bazel
load(":defs.bzl", "pycross_wheel_build_3_10")

pycross_wheel_build_3_10(
    name = "zstandard_build",
    config_settings = {
        "--build-option": [
            "--no-cffi-backend",
            "--system-zstd",
        ],
    },
    copts = ["-Wl,-s"],
    native_deps = [
        "//third_party/zstd",
    ],
    post_build_hooks = [
        "@rules_pycross//pycross/hooks:repair_wheel",
    ],
    sdist = "@uv//zstandard:sdist",
    tags = ["manual"],
    deps = [
        "@uv//:setuptools",
        "@uv//:wheel",
    ],
)

Regarding the pycross toolchain, the transition does in fact work for that toolchain type. I didn't realise it uses the same config setting as rules_python. So that's definitely a win.