* Use the newer "pyproject.toml" for syntax highlighters
* Use venv for everything (previously, the pygment lexers were installed
globally, but more recent Python releases frown upon that. This will probably
be more stable in the long run...)
* Use one venv at the root of the project (`_python_venv/`) for both
pygments and the runtime
* Initialise this venv automatically from the Makefile (it should no longer be
necessary to run scattered `set_up.sh` scripts with mysterious interactions)
On the downsides, though:
* Only tested with Python 3.11 at the moment
* Need to remember to activate the venv (`. _python_venv/bin/activate`) from the
root for e.g. HTML literate output to work. A more manageable solution could
be for Catala to provide the lexers on-the-fly when calling `pygmentize`.
