pyo3: make //toolchains:pyo3 publicly visible

[georgew-arta]

//toolchains:pyo3 (the rust_library_group wrapping @rpyo3c//:pyo3) is the natural dependency for plain rust_library crates that link against pyo3 outside of a pyo3_extension — e.g. a shared bridge library consumed by several extensions.

Today the target is package-private, so consumers have to either hard-code the bzlmod canonical repo label (@@rules_rust_pyo3~~rust_ext~rpyo3c//:pyo3) or use_repo the internal rust_ext extension from //private:internal_extensions.bzl — both couple downstream workspaces to implementation details (canonical-name mangling, internal extension paths) that change across Bazel releases.

This makes the existing target publicly visible so it can serve as the supported label, one line:

rust_library_group(
    name = "pyo3",
    visibility = ["//visibility:public"],
    deps = ["@rpyo3c//:pyo3"],
)

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