Stop installing C headers on PyPy

Fixes #39.

CHANGES.rst

@@ -5,7 +5,9 @@
 4.3.5 (unreleased)
 ==================
 
-- Nothing changed yet.
+- Stop installing C header files on PyPy (which is what zope.proxy before 4.3.4
+  used to do), fixes `issue 39
+  <https://github.com/zopefoundation/zope.proxy/issues/39>`_.
 
 
 4.3.4 (2020-03-13)

setup.py

@@ -32,7 +32,6 @@
 from setuptools import setup
 
 
-
 class optional_build_ext(build_ext):
     """This class subclasses build_ext and allows
        the building of C extensions to fail.
@@ -75,8 +74,10 @@ def read(*rnames):
 is_pypy = platform.python_implementation() == 'PyPy'
 if is_pypy:
     ext_modules = []
+    headers = []
 else:
     ext_modules = codeoptimization
+    headers = [os.path.join('src', 'zope', 'proxy', 'proxy.h')]
 
 setup(name='zope.proxy',
       version='4.3.5.dev0',
@@ -115,7 +116,7 @@ def read(*rnames):
       cmdclass={
           'build_ext': optional_build_ext,
       },
-      headers=[os.path.join('src', 'zope', 'proxy', 'proxy.h')],
+      headers=headers,
       ext_modules=ext_modules,
       install_requires=[
           'zope.interface',