check for build_ext object in import check

When setuptools imports from Cython (Python 3.5),
setuptools.command.build_ext is defined,
but setuptools.command.build_ext has no attribute .build_ext, yet,
causing the `_build_ext.build_ext` to fail with an AttributeError.

By attempting to import the object, rather than its containing module, the check is more thorough.

Cython/Distutils/build_ext.py

@@ -2,16 +2,16 @@ import sys
 
 if 'setuptools' in sys.modules:
     try:
-        from setuptools.command import build_ext as _build_ext
+        from setuptools.command.build_ext import build_ext as _build_ext
     except ImportError:
         # We may be in the process of importing setuptools, which tries
         # to import this.
-        from distutils.command import build_ext as _build_ext
+        from distutils.command.build_ext import build_ext as _build_ext
 else:
-    from distutils.command import build_ext as _build_ext
+    from distutils.command.build_ext import build_ext as _build_ext
 
 
-class build_ext(_build_ext.build_ext, object):
+class build_ext(_build_ext, object):
     def finalize_options(self):
         if self.distribution.ext_modules:
             from Cython.Build.Dependencies import cythonize