Fix wheel building for PyPy.

Don't confuse ext_modules with headers.

Fixes #88

.travis.yml

@@ -60,6 +60,8 @@ install:
     - pip install -U -e .[test]
 script:
     - python --version
+    # make sure we can build a wheel
+    - python setup.py bdist_wheel
     # coverage makes PyPy run about 3x slower, but the tests only take
     # .4s to begin with (the whole process takes about 1.5), so that's
     # still only 4.5s, which is maneagable.

CHANGES.rst

@@ -4,7 +4,8 @@
 4.4.1 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- Fix installation of source packages on PyPy. See `issue 88
+  <https://github.com/zopefoundation/persistent/issues/88>`_.
 
 
 4.4.0 (2018-08-22)

setup.py

@@ -39,7 +39,10 @@ is_jython = 'java' in sys.platform
 # anti-optimizations (the C extension compatibility layer is known-slow,
 # and defeats JIT opportunities).
 if is_pypy or is_jython:
-    ext_modules = headers = []
+    # Note that all the lists we pass to setuptools must be distinct
+    # objects, or bad things happen. See https://github.com/zopefoundation/persistent/issues/88
+    ext_modules = []
+    headers = []
 else:
     ext_modules = [
         Extension(