fix examples/KML.ipynb to work with recent versions of geopandas

[rdhyee]

examples/KML.ipynb is broken in v 0.17.4 of ipyleaflet -- you can verify this problem by running the notebook in mybinder.org and see the following error:

I implemented a fix based on the discussion at geopandas - AttributeError: 'NoneType' object has no attribute 'drvsupport' when using Fiona driver - Stack Overflow.

You can also see how this PR fixes the problem by running the notebook in this branch on mybinder.org:

[rdhyee]

Curious whether the document build failure is related to the changes in how Stamen is hosting its tiles.

[martinRenou]

Triggering CI