Force cython when building sdist

Fixes #182

setup.py

diff --git a/setup.py b/setup.py
index a1a87a2..cc102d9 100644
--- a/setup.py
+++ b/setup.py
@@ -77,12 +77,16 @@
     sys.modules['distutils.command.build_ext'].Extension = _Extension
 
 with_cython = False
+if 'sdist' in sys.argv:
+    # we need cython here
+    with_cython = True
 try:
     from Cython.Distutils.extension import Extension as _Extension
     from Cython.Distutils import build_ext as _build_ext
     with_cython = True
 except ImportError:
-    pass
+    if with_cython:
+        raise
 
 try:
     from wheel.bdist_wheel import bdist_wheel