## Summary Stacked on #2932. After the rustc bump, `snap-build` gets past `maturin` and fails on `protobuf`: ``` ModuleNotFoundError: No module named 'pkg_resources' ERROR: Failed to build 'protobuf' when getting requirements to build wheel ``` The snap builds everything from source (`PIP_NO_BINARY: ":all:"`), so pip also creates protobuf's isolated build environment. protobuf 3.20.3's `setup.py` imports `pkg_resources`, which setuptools removed in 81; current setuptools is 84, so that sdist cannot build. pip also ignores constraints files for build-isolation, so the build environment cannot be pinned from the repo. protobuf 3.20.3 -> 4.25.8 is API compatible: the 4.x line of that era is a re-versioning of the 3.20 code, and `macaroonbakery` (the only consumer) just requires `protobuf >=3.20.0`. Verified locally that the 4.25.8 sdist builds fully on Python 3.10 with setuptools 84. The pure-python `protobuf-3.20.3-py2.py3-none-any.whl` could be pulled in with `PIP_ONLY_BINARY: protobuf` to keep the old version, but that ships the slower pure-Python implementation to avoid a version bump, so it was rejected. > [!NOTE] > Replaces #2934, whose base branch was removed when the two windows-build PRs > were merged into #2931. Same branch, same commit. ## Test plan CI: `snap-build` completes the `charmcraft` part with `protobuf` compiled from source, which unblocks `snap-tests`. > [!WARNING] > Generated by an automated agent, not a human. 🤖 **BEEP BOOP I'M A TOASTER** _Model:_ `step-5-preview-free` _Harness:_ opencode CLI