setup.py: ~fix "rocksdb" extra by pinning old Cython version

see twmht/python-rocksdb#108

Author: SomberNight

setup.py

@@ -29,7 +29,7 @@ def find_version():
     extras_require={
         'dev': ['objgraph'],
         'rapidjson': ['python-rapidjson>=0.4.1,<2.0'],
-        'rocksdb': ['python-rocksdb>=0.6.9'],
+        'rocksdb': ['python-rocksdb>=0.6.9', 'Cython<3.0'],
         'ujson': ['ujson>=2.0.0,<4.0.0'],
         'uvloop': ['uvloop>=0.14'],
         # For various coins