every package in the monorepo did indeed need to use the same exact version of python. the python interpreter itself is part of the monorepo, and all other python code builds and runs that binary.
the python team did not personally upgrade every line of code to work with a new python version, we made sure that the code could be upgraded and got the people maintaining each package to do the upgrade. (a missing piece of the puzzle is that every package, even mirrored third party code, has an official owner responsible for keeping it working within google).
what the python team needed to do was upgrade all the tooling (build system, linter, type checker, etc) to work with the new version, run large scale tests to see what would break in an upgrade, analyse the dependency graph to figure out what order packages needed to be fixed in (e.g. if numpy needed to be upgraded that would be super high priority because thousands of other packages depend on it, and we might even pitch in and help the team maintaining it with that if needed), and track the progress of the upgrades, communicating with teams whose code hadn't been upgraded so that they could prioritise the work.
sometimes we also needed to write automated refactorings to fix some basic code pattern that changed, e.g. if some very widely used library decided it was going to stop accepting numbers where it needed strings, and that the caller should do the conversion first, we would try to mechanically fix that across the codebase rather than make everyone do it themselves.
anyway, this is all to say that it was indeed a large and time consuming problem, and we were certainly not doing it because we had nothing better to do with our time.