That's how transitions for languages like python (and ruby, and others) are handled: 1) add support for the new version, and rebuild stuff to be able to run on multiple versions 2) switch the default version 3) drop support for the old version. You filed this bug while we were in the middle of the process. Since a couple of weeks ago support for the old version (3.5) was removed, a rebuild of numpy has been issued and now it only depends on python 3.6.
My point is that even during such a transition, there is no need to require the user to have multiple versions of Python installed in order to be able to install the package. Even if python3-numpy is built to work on both python3.5 and python3.6, the user should be able to install python3-numpy even if only one of the Python versions is installed. Now that python3-numpy is built against python3.6 only, this is not relevant, of course.
Closing.
