[slides] Add remark for Ubuntu venv

slides/python.md

@@ -15,6 +15,7 @@ title: Python bindings
 * Enabled and built by default!
 * `dune-common` configures a [Python virtual environment](https://docs.python.org/3/library/venv.html) (`dune-common/build-cmake/dune-env`)
 * Automatically created and set up when running `dunecontrol`
+* On Ubuntu systems you need to install `python3-venv` (e.g. `sudo apt install python3-venv`) for `venv` to work properly.
 * To activate:
   ```sh
   source dune-common/build-cmake/dune-env/bin/activate