remove [all] extra from ipython, and add comment explaining fiona pin

Author: emilykl

test_requirements/requirements_optional.txt

@@ -1,18 +1,18 @@
 requests
 pandas
 numpy
-fiona<=1.9.6;python_version<="3.8"
 xarray
 statsmodels
 Pillow
 pytest
 pytz
-ipython[all]
+ipython
 ipykernel
 jupyter
 scipy
 Shapely
 geopandas
+fiona<=1.9.6;python_version<="3.8"  # fiona>1.9.6 is not compatible with geopandas<1; geopandas>=1 is not compatible with python 3.8
 pyshp
 matplotlib
 scikit-image
@@ -23,4 +23,4 @@ polars[timezone]
 pyarrow
 anywidget
 plotly-geo
-vaex;python_version<="3.9"
+vaex;python_version<="3.9"