Issue: PEP 461 style bytes formatting (b'%b' % variable) returns basestring. So the following code results in a cythonize error: ```cython cpdef bytes cy_foo(): cdef: bytes bytes_1, bytes_2 bytes_1 = b'bytes 1' bytes_2 = b'bytes 2' return b'%(bytes_1)b %(bytes_2)b' % { b'bytes_1': bytes_1, b'bytes_2': bytes_2} ``` Result: `Cannot convert 'basestring' object to bytes implicitly. This is not portable.` This same basic code in python 3.5.1+ returns a bytes object.