The Python xasm module has routines for assembly, and has a command to assemble
bytecode for several different versions of Python.

Here are some potential uses:
- Make small changes to existing Python bytecode when you don't have source
- Craft custom and efficient bytecode
- Write an instruction-level optimizing compiler
- Experiment with and learn about Python bytecode
- Foil decompilers like uncompyle6 so that they can't disassemble bytecode (at
  least for now)

This support the same kinds of bytecode that xdis supports. This is pretty much
all released bytecode, although we tend to lag behind the latest Python
releases.