Inadequate dev env setup instructions

[scop]

Our dev env setup instructions are inadequate, indeed a python virtualenv must be set up in the bash-completion working dir, then the above should work. Basically the commands in this run section need to be run locally, or replacing the first two with a functionally equivalent pyenv and pyenv-virtualenv setup (that's what I'm using), venv-run then picks the env up and things should work in principle. The other mypy related issue you mentioned is something different, this doesn't fix that one, but I don't think I'm hitting it every time.

Originally posted by @scop in #547 (comment)