consider dropping python dev packages

Consider dropping the Python 2 and 3 dev packages from the container. Would save about 120 MB, about 10%.

Need to test whether OctSymPy CI still works without python dev packages installed.

Projects that need to link against Python can install as needed, or use conda or pyenv. I'm considering using pyenv for Pythonic CI anyway to be able to test multiple Pythons, not just whatever Ubuntu provides.