fix: Use non-gui matplotlib backend for tests to avoid hanging on ipywidgets interact (#2098)

* Avoid the notebook tests hanging on the ipywidgets interact sections of `InterpolationCodes.ipynb`
  by setting the matplotlib backend to the non-gui Agg which disallows the creation of the gui elements
  that are causing the problem.
* Remove '%pylab inline' from InterpolationCodes.ipynb as pylab is very deprecated.
* Add --verobse flag to notebook tests to make it easier to track the progress of the notebooks in the
  CI logs.

Author: matthewfeickert

.github/workflows/notebooks.yml

@@ -39,4 +39,4 @@ jobs:
         # Override the ini option for filterwarnings with an empty list to disable error
         # on filterwarnings as testing for notebooks to run with the latest API, not if
         # Jupyter infrastructure is warning free.
-        pytest --override-ini filterwarnings= tests/test_notebooks.py
+        pytest --verbose --override-ini filterwarnings= tests/test_notebooks.py

docs/examples/notebooks/learn/InterpolationCodes.ipynb

@@ -4,19 +4,11 @@
    "cell_type": "code",
    "execution_count": 1,
    "metadata": {},
-   "outputs": [
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "Populating the interactive namespace from numpy and matplotlib\n"
-     ]
-    }
-   ],
+   "outputs": [],
    "source": [
-    "%pylab inline\n",
-    "from ipywidgets import interact\n",
     "import matplotlib.pyplot as plt\n",
+    "import numpy as np\n",
+    "from ipywidgets import interact\n",
     "from mpl_toolkits.mplot3d import Axes3D"
    ]
   },

tests/test_notebooks.py

@@ -1,10 +1,14 @@
+import os
 import sys
 from pathlib import Path
 
 import papermill as pm
 import pytest
 import scrapbook as sb
 
+# Avoid hanging on with ipywidgets interact by using non-gui backend
+os.environ["MPLBACKEND"] = "agg"
+
 
 @pytest.fixture()
 def common_kwargs(tmpdir):