Fix lumino message handling (context menu)

[vidartf]

Without this compatibility shim (https://ipywidgets.readthedocs.io/en/stable/migration_guides.html#phosphor-lumino), we no longer handle contextmenu events correctly (with ipywidgets 8).

Fixes #392.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.54%. Comparing base (7dacddf) to head (99f9a9a).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #397 +/- ## ======================================= Coverage 70.54% 70.54% ======================================= Files 23 23 Lines 893 893 ======================================= Hits 630 630 Misses 263 263 

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