protobuf: build with up-to-date dateutil

Fixes #32571. @mistydemeo, can you take a look / try this?
tl;dr: won't work.

There are two cases, apps and libraries.

Apps, like ansible, don't need to be `import`able after they're installed. Python module dependencies of apps can be installed into libexec where they won't affect the rest of the system; we tell apps where to find its dependencies by setting PYTHONPATH at runtime with an env_script. In this case, Python dependencies can be installed unconditionally.

Libraries and all of their dependencies need to be `import`able without setting a formula-specific PYTHONPATH, which means they need to be present in or linked to HOMEBREW_PREFIX, so we install into prefix instead of libexec. But if a user has already `pip install`ed a Python package  into HOMEBREW_PREFIX and then we install the same package into a formula's `prefix`, `brew link` will fail because the files conflict.

There are alternatives; my pet idea at present is installing python everything into libexec and then dropping a e.g. homebrew-protobuf.pth file into lib/.../site-packages pointing to the formula's libexec and linking that instead, which should never cause link failures, is more reproducible, eliminates problems with stale .pyc files in HOMEBREW_PREFIX, and will supplement the packages the user already has. But it depends on the HOMEBREW_PREFIX site-packages being a special site-packages folder, and we don't give system Python users advice on how to do that. But we probably should, à la #30844.