Hi Andrew,
This is weird -- it should not have been able to compile and link the Python interface to igraph without libxml2 at all.
You mentioned that you installed python-igraph from pip -- this almost surely means that igraph itself was compiled on your machine locally. The only thing that I can think of right now is that you have libxml2 on your machine but not libxml2-dev; the latter package is needed to _compile_ packages that link to libxml2. So, I would probably try this:
- ensure that libxml2-dev and pkg-config are both installed
- uninstall python-igraph (either destroy the venv and start from scratch, or run pip uninstall python-igraph)
- reinstall python-igraph with "pip install python-igraph"
All the best,