Since you mention Jessie, I assume you've updated your Ultra4 to OS6. This guy did successfully install Python 3.7 on a generic Jessie system: https://jameskiefer.com/posts/installing-python-3.7-on-debian-8/ . But that was before Jessie was archived, so there may be other hurdles now. One potential issue is that it needs OpenSSL 1.0.2 or 1.1 and OS6 has 1.0.1. But upgrading that to 1.0.2, as the poster of that article did, could break something in OS6.
