Tom Ritter:
The requirement will likely be Python 3.5+. Although I would love to make that 3.6 if possible so we can fully harness modern features and performance.
I'm too lazy to check what's the base OS for the Tor Browser build boxes but here's what we have in Debian:
python3.5 | 3.5.3-1 | stable | source, amd64, i386 python3.5 | 3.5.4-4 | testing | source, amd64, i386 python3.5 | 3.5.4-4 | unstable | source, amd64, i386 python3.6 | 3.6.3-1 | testing | source, amd64, i386 python3.6 | 3.6.3-1 | unstable | source, amd64, i386
So as long as Tor Browser is built on Debian 9 (Stretch aka. stable) or newer, we're good. Debian 8 (Jessie aka. oldstable) only has 3.4 so it won't cut it.
(In passing: Tom, from my — somewhat remote — perspective, you're doing an outstanding liaison job with Mozilla. Many thanks!)