Revert "raise cython version"

This reverts commit a0a0e2c.

Author: Jonathan Kliem

setup.py

@@ -66,7 +66,7 @@ def run(self):
 setup(
     name='cypari2',
     version=VERSION,
-    setup_requires=['Cython>=0.29.28'],
+    setup_requires=['Cython>=0.28'],
     install_requires=['cysignals>=1.7'],
     description="A Python interface to the number theory library PARI/GP",
     long_description=README,