Use pure Python Rtree library

The Python rtree library we are using to build RTrees has a dependency
on the C package libspatialindex. This package is not installed on PaaS,
so it’s hard for us to use it.

This commit changes the code to use a library called rtreelib instead.

rtreelib doesn’t have a built in way to serialise the index it builds,
so I’ve had to implement that using pickle.

requirements.in

@@ -19,7 +19,7 @@ gunicorn==20.1.0
 eventlet==0.30.2
 notifications-python-client==6.0.2
 Shapely==1.7.1
-Rtree==0.9.7
+rtreelib==0.2.0
 
 # PaaS
 awscli-cwlogs>=1.4,<1.5