Freeze requirements, fix test script.

noxfile.py

+""" Run notebook tests in a clean environment using requirements.txt. """
+
 import nox
 
 
 @nox.session(python=["3.9"], reuse_venv=True)
 def nbtest(session):
     session.install("-r", "requirements.txt")
-    session.run("bash", "test.sh", ".", external=True)
+    session.run("bash", "test.sh", "Machine-Learning", "Pandas-DataFrames", "Python-Jupyter", "Visualisation", external=True)

requirements.txt

-IPython
 jupyter
-matplotlib
-numpy
-pandas
-pydicom
-scipy
-seaborn
-scikit-image
-scikit-learn
+ipython
+matplotlib==3.4.2
+numpy==1.21.0
+pandas==1.3.0
+scipy==1.7.0
+seaborn==0.11.1
+scikit-image==0.18.2
+scikit-learn==0.24.2

test.sh

 #!/bin/bash
 
+# Try executing all notebooks and converting to html. Catches formatting and
+# execution errors. Note that notebooks which intentionally have error cells
+# won't make the 'ok' list. Run this using 'nox' to get the environment right.
+
+# Find failures after the run using
+#
+#    diff <(sort ok.lst) <(sort tested.lst) | egrep "^>"
+#
+
 rm -f tested.lst
-git ls-files $1 | grep ipynb | xargs -I {} sh -c 'echo {} >> tested.lst; if jupyter nbconvert --to html --execute {}; then echo {}; fi' > ok.lst
+git ls-files $@ | grep ipynb | xargs -I {} sh -c 'echo {} >> tested.lst; if jupyter nbconvert --to html --execute {}; then echo {}; fi' > ok.lst