Add unit testing for multiple python versions in `multipy/runtime`

[PaliC]

We should add unit testing for explicitly using python interpreters from python versions which are not python 3.8.6, so we can detect if a change in python breaks something in multipy. Furthermore, the added testing covers usecases of downstream customers who do not use 3.8.6.

Dependencies:

• More generally support python in Multipy #96
• Using pyenv to dynamically link to python in multipy/runtime #97