title: build sklearn error cythonize failed
body: <pre><code> $ sudo python setup.py build
  </code></pre>
  
  <p>Gives me the following:</p>
  
  <pre><code>Partial import of sklearn during the build process.
  Generating cython files
  Cythonizing sources
  sklearn
  Processing sklearn/_isotonic.pyx
  Traceback (most recent call last):
    File "/home/scikit-learn/build_tools/cythonize.py", line 198, in &lt;module&gt;
      main(root_dir_arg)
    File "/home/ascikit-learn/build_tools/cythonize.py", line 190, in main
      check_and_cythonize(root_dir)
    File "/home/scikit-learn/build_tools/cythonize.py", line 182, in check_and_cythonize
      cythonize_if_unchanged(cur_dir, cython_file, gen_file, hashes)
    File "/home/scikit-learn/build_tools/cythonize.py", line 156, in cythonize_if_unchanged
      cythonize(full_cython_path, full_gen_file_path)
    File "/home/scikit-learn/build_tools/cythonize.py", line 61, in cythonize
      raise Exception('Building scikit-learn requires Cython &gt;= 0.21')
  Exception: Building scikit-learn requires Cython &gt;= 0.21
  Traceback (most recent call last):
    File "setup.py", line 297, in &lt;module&gt;
      setup_package()
    File "setup.py", line 280, in setup_package
      generate_cython()
    File "setup.py", line 185, in generate_cython
      raise RuntimeError("Running cythonize failed!")
  RuntimeError: Running cythonize failed!
  </code></pre>
  
  <p>While cython is installed:</p>
  
  <pre><code>Processing ./Cython-0.24-cp27-cp27mu-manylinux1_x86_64.whl
  Installing collected packages: Cython
    Found existing installation: Cython 0.24
      Uninstalling Cython-0.24:
        Successfully uninstalled Cython-0.24
  Successfully installed Cython-0.24
  </code></pre>