build(deps-dev): update sphinx-external-toc requirement from ~=0.2.3 to ~=0.3.1 (#1887)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

pyproject.toml

@@ -38,7 +38,7 @@ dependencies = [
     "sphinx>=4,<5",
     "sphinx-comments",
     "sphinx-copybutton",
-    "sphinx-external-toc~=0.2.3",
+    "sphinx-external-toc~=0.3.1",
     "sphinx-jupyterbook-latex~=0.4.6",
     "sphinx-design~=0.1.0",
     "sphinx-thebe~=0.2.1",