EXPERIMENT: play with cython's dependency resolution

Patching up Cython.Build via the included cython.patch,
then running `python setup.py build_ext -i`, and walking the stack in pdb,
can get:

(Pdb) p deps.cimported_files('observable.pyx')
('observable.pxd',)
(Pdb) p deps.included_files('observable.pyx')
{'foobar.pxi'}

So that should be enough to cook up a depfile for `observable.pyx` from this?

mc_lib/cython.patch

@@ -0,0 +1,15 @@
+$ git diff
+diff --git a/Cython/Build/Dependencies.py b/Cython/Build/Dependencies.py
+index ae491c7c6..9f863b2b7 100644
+--- a/Cython/Build/Dependencies.py
++++ b/Cython/Build/Dependencies.py
+@@ -990,6 +990,8 @@ def cythonize(module_list, exclude=None, nthreads=0, aliases=None, quiet=False,
+     deps = create_dependency_tree(ctx, quiet=quiet)
+     build_dir = getattr(options, 'build_dir', None)
+
++    import pdb; pdb.set_trace()
++
+     def copy_to_build_dir(filepath, root=os.getcwd()):
+         filepath_abs = os.path.abspath(filepath)
+         if os.path.isabs(filepath):
+

mc_lib/foobar.pxi

@@ -0,0 +1,3 @@
+
+def foo():
+    return None

mc_lib/observable.pyx

@@ -8,6 +8,8 @@ import numpy as np
 from .observable cimport ScalarObservable
 from .observable cimport trampoline_mrg
 
+include "foobar.pxi"
+
 cdef class RealObservable():
 
     def __cinit__(self):

mc_lib/setup.py

@@ -0,0 +1,6 @@
+from setuptools import setup
+from Cython.Build import cythonize
+
+setup(
+    ext_modules = cythonize("observable.pyx")
+)