fix: Add ipympl to 'test' extra (#2255)

* Add ipympl>=0.3.0 to the 'test' extra environment as it is used in the
  notebooks as of PR #2252. The lower bound of v0.3.0 is chosen as this
  corresponds in time with the 'contrib' lower bound on matplotlib of
  matplotlib>=3.0.0.
* Amends PR #2252.

pyproject.toml

@@ -96,6 +96,7 @@ test = [
     "pytest-benchmark[histogram]",
     "pytest-console-scripts>=1.4.0",
     "pytest-mpl",
+    "ipympl>=0.3.0",
     "pydocstyle",
     "papermill~=2.3.4",
     "scrapbook~=0.5.0",