Use venv for tests, pipx for install

[rkent]

This approach to fixing the issues in Ubuntu 24 is to use pipx for a normal installation of rosdoc2, but tests using venv.

Using pipx required some changes to sphinx_builder. The venv that is setup behind the scenes for pipx did not translate into a python subprocess, so runs of sphinx-build failed because of a missing exhale dependency. The solution to that was to follow the same process I do in tests, namely to run sphinx directly as a python module and not as a subprocess. That did not seem to be needed for sphinx-apidoc.

pipx has enough issues that it was worth adding directly to the test suite, simulating what we expect for user install.

[rkent]

Just a note. This patch is more of a demonstration of an alternate approach than a serious request to land code. The larger ROS2 project will need to decide if PIP_BREAK_SYSTEM_PACKAGES: 1 is the recommendation to users, or if the use of venv and pipx, as recommended in pip's "Externally Managed Environments" is to be followed.

What this patch demonstrates is that rosdoc2 can, with minor changes, be used with pipx. We should probably just do those changes anyway regardless of the overall project decision.

[tfoote]

We should probably converge our systems to using pipx in the future. For the moment I'd suggest we hold off until we're ready to do it more generally.

[rkent]

I'm going to close this, but rework it to only include the changes (and tests) for pipx compatibility. I'll explain why in a new PR.