Downgrade version number to 1.21a15, it was never released

Author: Photonios

setup.py

@@ -34,7 +34,7 @@ def run(self):
 
 setup(
     name='django-postgres-extra',
-    version='1.21a16',
+    version='1.21a15',
     packages=find_packages(),
     include_package_data=True,
     license='MIT License',