Unpin xtl in the tests

[jtpio]

In #808, we pinned to xtl=0.6.23 to fix the tests (xeus-cling depends on xtl):

voila/.github/workflows/main.yml

Line 34 in 3209c1f

run: mamba install -q python=${{ matrix.python_version }} pip jupyterlab_pygments==0.1.0 pytest-cov nodejs flake8 ipywidgets matplotlib xeus-cling xtl=0.6.23

From #808 (comment)

The xeus-cling stack must be rebuilt so it uses the same standard lib as the last xtl

This issue is a reminder to unpin at some point in the future.