Issue 2125 broken selenium tests (#2129)

* Add a version fix for jupytext

* Add a comment to explain

Author: hansthen

requirements-dev.txt

@@ -16,7 +16,7 @@ gpxpy
 ipykernel
 jenkspy
 jupyter
-jupytext
+jupytext==1.16.*   # Due to selenium test failures in 1.17.0.
 matplotlib
 mypy
 nbconvert