Ubuntu – Install applications in Ubuntu core

If what you want is a development environment, then you should check out this answer, about the classic snap, which is to provide a chroot with a classic console development experience. You can then install Python 3.6 normally through packages or by building from source, here, to develop with.

If however, what you want is a specific Python version to depend on in a package which you intend to ship and run on Ubuntu core, you will need to build that specific version of Python, and any additional libraries you may need for it, and include it within your snap package.