fix: add Voila >= 0.2.8 support

[mariobuikhuizen]

No description provided.

[havok2063]

Presumably this would also address #552?

[ibusko]

I verified it works with 0.2.8. It also seems to fix #552

[mariobuikhuizen]

I don't know if it fixes #552, since the reporter of the issue said it was not solved by downgrading Voila to 0.2.7.

[havok2063]

@mariobuikhuizen sometimes I've found that a voila version alone isn't enough to solve an issue. That we have have to pin specific versions of dependencies, e.g. traitlets or jupyter-server. Could that be an issue in the user's case? What dependency versions should we make sure users have for voila 0.2.7 or above?