Resolving toga-gtk attempts to build pygobject on MacOS

[ncoghlan]

toga-gtk fails to generate a valid cross-platform lock file when added as a dependency on platforms other than Linux, as pygobject can only be published as an sdist: beeware/toga#2995

(statically linking the GObject bindings is not desirable, and a valid manylinux wheel can't just assume those libraries exist)

Setting tool.uv.dependency.metadata doesn't solve the problem, since that's only effective for application locks - the tool table metadata doesn't make it into the toga-gtk .dist-info folder, so uv add doesn't have access to it when the dependency on toga-gtk is resolved.

Searching the code for build_wheel and prepare_metadata_for_build_wheel, it isn't clear to me if the problem is that uv simply isn't trying the latter option before progressing to the full wheel build, or if the latter isn't working with meson-build, so uv is falling back to the former, which than fails due to the platform incompatibility.

Potentially related:

• architecture mismatch issue: Build backend failed to build wheel through build_wheel() - wrong architecture #3308
• meson-build issue: uv doesn't play well with meson-python build backend and pytest #8966