Bump pillow from 7.1.2 to 8.1.1 in /example (#317)

Bumps [pillow](https://github.com/python-pillow/Pillow) from 7.1.2 to 8.1.1.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@7.1.2...8.1.1)

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example/requirements.txt

@@ -2,4 +2,4 @@ Django==2.2.18
 django-geojson==2.12.0
 django-leaflet==0.26.0
 jsonfield==2.0.2
-Pillow==7.1.2
+Pillow==8.1.1