The unpacker currently requires the file to return bytes.

cdef read_from_file(self):
 next_bytes = self.file_like_read(
 min(self.read_size,
 self.max_buffer_size - (self.buf_tail - self.buf_head)
 ))
 if next_bytes:
 self.append_buffer(PyBytes_AsString(next_bytes), PyBytes_Size(next_bytes))
 else:
 self.file_like = None

Is it possible to relax this?

The returned object must of course support the buffer protocol, but beyond that it shouldn't be required to be a bytes type. It could be a memoryview into an in-memory "file". There should be no requirement to copy the data; the unpacker does that anyway.