makeopcodetargets.py: we need to import Lib/opcode.py

Issue #20021: use importlib.machinery to import Lib/opcode.py and not an opcode
module coming from somewhere else. makeopcodetargets.py is part of the Python
build process and it is run by an external Python program, not the built Python
program.

Patch written by Serhiy Storchaka.