make cygrpc importable on py3

setup.py

@@ -42,6 +42,7 @@ from setuptools.command import egg_info
 # Redirect the manifest template from MANIFEST.in to PYTHON-MANIFEST.in.
 egg_info.manifest_maker.template = 'PYTHON-MANIFEST.in'
 
+PY3 = sys.version_info.major == 3
 PYTHON_STEM = './src/python/grpcio'
 CORE_INCLUDE = ('./include', '.',)
 BORINGSSL_INCLUDE = ('./third_party/boringssl/include',)
@@ -103,7 +104,11 @@ if "linux" in sys.platform:
   LDFLAGS += ('-Wl,-wrap,memcpy',)
 if "linux" in sys.platform or "darwin" in sys.platform:
   CFLAGS += ('-fvisibility=hidden',)
-  DEFINE_MACROS += (('PyMODINIT_FUNC', '__attribute__((visibility ("default"))) void'),)
+
+  pymodinit_type = 'PyObject*' if PY3 else 'void'
+
+  pymodinit = '__attribute__((visibility ("default"))) {}'.format(pymodinit_type)
+  DEFINE_MACROS += (('PyMODINIT_FUNC', pymodinit),)
 
 
 def cython_extensions(package_names, module_names, extra_sources, include_dirs,