Adds cythonize() call to extensions in setup.py()

I've been having trouble build hdbscan on MacOS with normal
python (e.g. outside of conda). It turns out that nothing was
instructing the build system to actually compile the cython files.

After reading the cython documentation, it seems like the easiest
fix is to wrap the extensions list in cythonize(). Locally this
built fine.

Author: axiak

setup.py

@@ -8,6 +8,7 @@
     HAVE_CYTHON = True
 except ImportError as e:
     warnings.warn(e.args[0])
+    cythonize = lambda ext: ext
     from setuptools import setup, Extension
     from setuptools.command.build_ext import build_ext
     HAVE_CYTHON = False
@@ -42,6 +43,7 @@ def run(self):
                          sources=['hdbscan/dist_metrics.pyx'])
 
 
+
 def readme():
     with open('README.rst') as readme_file:
         return readme_file.read()
@@ -78,12 +80,13 @@ def requirements():
     'license': 'BSD',
     'packages': ['hdbscan', 'hdbscan.tests'],
     'install_requires': requirements(),
-    'ext_modules': [_hdbscan_tree,
+    'ext_modules': cythonize([
+                    _hdbscan_tree,
                     _hdbscan_linkage,
                     _hdbscan_boruvka,
                     _hdbscan_reachability,
                     _prediction_utils,
-                    dist_metrics],
+                    dist_metrics]),
     'cmdclass': {'build_ext': CustomBuildExtCommand},
     'test_suite': 'nose.collector',
     'tests_require': ['nose'],