It’s good to see you again!
Python 3.7.4 is in OpenWrt 19.07, which was not released and there is no RC release, yet. If you would like to try that, you can. In branch HBD, you will find OpenWrt 19.07 (Turris OS 5.x). You can find it also in OpenWrt snapshots, which are daily built on the latest commits in master branches. We have them also and you can try to use master branch in switch-branch.
In OpenWrt 18.06.04, there’s Python 3.6.9. The same version is included in Turris OS 4.0 since beta7 version, which was released mostly because CVE-2018-20852, which was found in Python, and while I was doing it, I updated them to their latest versions in OpenWrt as we take most packages including Python as they are from OpenWrt.
As you are asking about Turris OS 3.x release, if I am not mistaken we wanted to update it as well, but when @paja was trying it, it doesn’t end up well.
If you would like to try newer version than the version of Python which is in Turris OS 3.x, I suggest you try current beta release of Turris OS 4.x.