core_.pyx: define basestring where not present (py3k)

gevent/core_.pyx

@@ -2,6 +2,7 @@
 cimport cython
 cimport libev
 from python cimport *
+import sys
 
 
 __all__ = ['get_version',
@@ -142,6 +143,12 @@ cpdef _flags_to_list(unsigned int flags):
     return result
 
 
+if sys.version_info[0] >= 3:
+    basestring = (bytes, str)
+else:
+    basestring = __builtins__.basestring
+
+
 cpdef unsigned int _flags_to_int(object flags) except? -1:
     # Note, that order does not matter, libev has its own predefined order
     if flags is None: