Improve nest package building with Python 3

While building Nest on Travis wee see:

On Darwin :

-- Configuring done
-- Generating done
-- Build files have been written to: /private/var/folders/my/m6ynh3bn6tq06h7xr3js0z7r0000gn/T/travis/spack-stage/spack-stage-rJw01S/nest-simulator-2.12.0/spack-build-2.12.0
==> 'make' '-j2'
Scanning dependencies of target generate_help
Extracting help information; this may take a litte while.
Traceback (most recent call last):
  File "/Users/travis/build/pramodskumbhar/spack-packages/spack/opt/spack/darwin-elcapitan-x86_64/clang-7.3.0-apple/py-cython-0.25.2-dys7mp2o2fr455jzg2v2riepllgwuhkr/lib/python3.6/site-packages/site.py", line 73, in <module>
    __boot()
  File "/Users/travis/build/pramodskumbhar/spack-packages/spack/opt/spack/darwin-elcapitan-x86_64/clang-7.3.0-apple/py-cython-0.25.2-dys7mp2o2fr455jzg2v2riepllgwuhkr/lib/python3.6/site-packages/site.py", line 3, in __boot
    import os
ImportError: No module named os
make[2]: *** [generate_help] Error 1
make[1]: *** [doc/CMakeFiles/generate_help.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Scanning dependencies of target nestutil

On Linux :

-- Host triple: x86_64-pc-linux
-- Target triple: x86_64-pc-linux
-- Found GSL: /usr/include (found version "1.16") 
-- Found PythonInterp: /opt/python/3.5.3/bin/python (found version "3.5.3") 
-- Found PythonLibs: /opt/python/3.5.3/lib/libpython3.5m.so (found suitable exact version "3.5.3") 
-- Found Cython: /usr/bin/cython  
CMake Error at cmake/ProcessOptions.cmake:375 (message):
  Your Cython version is too old.  Please install newer version (0.19.2+)
Call Stack (most recent call first):
  CMakeLists.txt:148 (nest_process_with_python)
-- Configuring incomplete, errors occurred!
==> Error: ProcessError: Command exited with status 1:
    'cmake' '..' '-DCMAKE_INSTALL_PREFIX:PATH=/home/travis/build/pramodskumbhar/spack-packages/spack/opt/spack/linux-ubuntu14-x86_64/gcc-4.8/nest-2.12.0-vxt3b37ola5ky763cm4hcq57iv2kuqdy' '-DCMAKE_C_COMPILER=/home/travis/build/pramodskumbhar/spack-packages/spack/lib/spack/env/gcc/gcc' '-DCMAKE_CXX_COMPILER=/home/travis/build/pramodskumbhar/spack-packages/spack/lib/spack/env/gcc/g++' '-Dwith-ltdl=OFF' '-Dwith-readline=OFF' '-Dwith-optimize=-g;-O3' '-Dwith-python=ON' '-Dcythonize-pynest=/usr' '-Dwith-gsl=/usr' '-Dwith-mpi=ON' '-Dwith-openmp=ON' '-Dstatic-libraries=OFF'
/home/travis/build/pramodskumbhar/spack-packages/packages/nest/package.py:136, in install:
     71       def install(self, spec, prefix):

BlueBrain / spack-packages

Improve nest package building with Python 3 #18