setup: do not build the C backend on PyPy

This should have always been the case. But it surprisingly worked up until
e5a3baf when a new header file referenced
CPython symbols not present in PyPy.

Closes #130.

docs/news.rst

@@ -76,6 +76,14 @@ Other Actions Not Blocking Release
 0.16.0 (not yet released)
 =========================
 
+0.15.1 (not yet released)
+=========================
+
+Bug Fixes
+---------
+
+* ``setup.py`` no longer attempts to build the C backend on PyPy. (#130)
+
 0.15.0 (released 2020-12-29)
 ============================
 

setup.py

@@ -8,6 +8,7 @@
 from __future__ import print_function
 
 from distutils.version import LooseVersion
+import platform
 import os
 import sys
 from setuptools import setup
@@ -52,6 +53,10 @@
 if os.environ.get("ZSTD_WARNINGS_AS_ERRORS", ""):
     WARNINGS_AS_ERRORS = True
 
+# PyPy doesn't support the C backend.
+if platform.python_implementation() == "PyPyy":
+    C_BACKEND = False
+
 if "--legacy" in sys.argv:
     SUPPORT_LEGACY = True
     sys.argv.remove("--legacy")