Use a fork of cython

The target commit removes the logging on contention
which slows the execution of the code.

kdtree/environment.yml

@@ -17,5 +17,5 @@ dependencies:
   - memory_profiler
   - pip:
       # Install cython+ from upstream directly
-      - -e git+https://lab.nexedi.com/nexedi/cython.git@fd3a224472d75f7c6107828c1e4b9587f3990a46#egg=Cython
+    - -e git+https://gitlab.inria.fr/jjerphan/cython.git@b30eafec6a7b174afdc4f023b45b21f85104e2fe#egg=Cython
       # The installation of the 'kdtree' module is made then