relocked examples env on python v3.6.15

https://github.com/fphammerle/freesurfer-surface/commit/8acc25505967ebbfc9baf0af500aa4b830d26917

.github/workflows/python.yml

@@ -149,6 +149,8 @@ jobs:
     # revert line specifying version of python interpreter
     - run: sudo apt-get update
     - run: sudo apt-get install --yes --no-install-recommends patchutils
-    - run: git diff --unified=0 | grepdiff --output-matching=hunk '^   "version":' | patch --reverse
-    - run: git diff --unified=0 | grepdiff --output-matching=hunk 'data:image/png;base64,' | patch --reverse
+    - run: >-
+        git diff --unified=0
+        | grepdiff --output-matching=hunk '^\(   "version":\|    "execution": {\|       "<img src=\\"data:image/png;base64,\)'
+        | patch --reverse
     - run: git diff --exit-code

examples/Pipfile

@@ -9,6 +9,8 @@ pandas = "*"
 matplotlib = "*"
 seaborn = "*"
 notebook = "*"
+# nbconvert v6.0.2 made `;` suffix on last line ineffective
+nbconvert = "<6.0.2"
 
 # workaround pipenv expectedly adding "python_version >= '3.7'" marker:
 # >   File "/home/runner/.local/share/virtualenvs/examples-bH_l4rXc/lib/python3.5/site-packages/nbconvert/exporters/base.py", line 10, in <module>