Prefer new_build_ext for Cython builds as recommended by Cython project. Fixes #1270.

CHANGES.rst

@@ -4,6 +4,9 @@ v38.5.0
 * #1229: Expand imports in ``build_ext`` to refine detection of
   Cython availability.
 
+* #1270: When Cython is available, ``build_ext`` now uses the
+  new_build_ext.
+
 v38.4.1
 -------
 

setuptools/command/build_ext.py

@@ -14,7 +14,7 @@ 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
+    from Cython.Distutils.build_ext import new_build_ext as _build_ext
     # Additionally, assert that the compiler module will load
     # also. Ref #1229.
     __import__('Cython.Compiler.Main')