A recent change added the use of subprocess.run(), which is 3.5 onwards. OE/bitbake currently supports 3.4 which is why I noticed (my CI bots use 3.4 to catch problems). There's no problem with deciding that 3.5 onwards is the requirement (apart from not being able to run on centos) but I think this needs to be decided and documented.
This should be resolved. We should be using the latest Python. I will check and then close.
Python 3.5 or later is the new default (I am running on Python 7 myself). The documentation is updated.