Import Cython.Compiler.Main as recommended by Cython project to improve Cython...

Import Cython.Compiler.Main as recommended by Cython project to improve Cython detection. Fixes #1229.

CHANGES.rst

+v38.5.0
+-------
+
+* #1229: Expand imports in ``build_ext`` to refine detection of
+  Cython availability.
+
 v38.4.1
 -------
 

setuptools/command/build_ext.py

@@ -15,6 +15,9 @@ from setuptools.extern import six
 try:
     # Attempt to use Cython for building extensions, if available
     from Cython.Distutils.build_ext import build_ext as _build_ext
+    # Additionally, assert that the compiler module will load
+    # also. Ref #1229.
+    __import__('Cython.Compiler.Main')
 except ImportError:
     _build_ext = _du_build_ext