If I remember correctly, there is a command-line flag for install.sh that allows you to point specifically to the python executable that you wish to use. I expect that the installer is defaulting to an installed Python 2.7 because up until very recently that was the most recent supported version of Python that worked with Plone. In fact, Plone 5.1 still only supports Python 2.7. The first version of Plone that will support Python 3 will be 5.2, which is not released yet.
You will likely have to re-build Python 2.7 with a new openssl in order to get things working properly. Alternatively, I believe the unified installers come with their own built-in Python that is properly tooled. At least, at one point this was the case. @tkimnguyen is this still true?