pythonPackages.Rtree: init at 0.8.3

This is a commonly-used package for spatial indexing.

Strangely enough, the tests are broken due to memory unsafety that I was unable
to reproduce under Debian. For instance, when run with Python 3.6 valgrind
reports,

==10453== Invalid read of size 4
==10453==    at 0x4EF1DFA: _PyObject_Free (in /nix/store/azw9ys2m2fpfzf730xjcxja890gpyp58-python3-3.6.4/lib/libpython3.6m.so.1.0)
==10453==    by 0x4FE7D96: _PyFaulthandler_Fini (in /nix/store/azw9ys2m2fpfzf730xjcxja890gpyp58-python3-3.6.4/lib/libpython3.6m.so.1.0)
==10453==    by 0x4F8438D: Py_FinalizeEx (in /nix/store/azw9ys2m2fpfzf730xjcxja890gpyp58-python3-3.6.4/lib/libpython3.6m.so.1.0)
==10453==    by 0x4F9F398: Py_Main (in /nix/store/azw9ys2m2fpfzf730xjcxja890gpyp58-python3-3.6.4/lib/libpython3.6m.so.1.0)
==10453==    by 0x400BDF: main (in /nix/store/azw9ys2m2fpfzf730xjcxja890gpyp58-python3-3.6.4/bin/python3.6)
==10453==  Address 0x67b1020 is 352 bytes inside a block of size 640 free'd
==10453==    at 0x4C2DD6B: fr...