Add a test to ensure that the GUI module is importable

This is an extremely basic check to catch errors like using incompatible
language features in GUI code which isn't imported by the CLI.
This is a temporary measure until we implement #24.

Author: maresb

.github/workflows/tests.yml

@@ -21,6 +21,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
+        sudo apt-get install -y libegl1
         python -m pip install --upgrade pip
         pip install tox tox-gh-actions
     - name: Test with tox

pyproject.toml

@@ -83,6 +83,7 @@ commands =
   pip freeze
   labelle --version
   labelle --help
+  python -c "import labelle.gui.gui; print('GUI import succeeded')"
   labelle --output console "single line"
   labelle --output console_inverted "inverted"
   labelle --output console multiple lines