setup: add --multi-version to the "setup.py develop" command-line

setup: add --multi-version to the "setup.py develop" command-line
fixes #530. I earlier tried this twice (see #530 for history) and then twice rolled it back due to some problems that arose. However, I didn't write down what the problems were in enough detail on the ticket that I can tell today whether those problems are still issues, so here goes the third attempt. (I did write down on the ticket that it would not create site.py or .pth files in the target directory with --multi-version mode, but I didn't explain why *that* was a problem.)