I fear the only solution is to quickly move to the Python 3 package that you already have prepared in experimental.
It's not quite that simple, first of all the python3-nototools package in experimental is rc buggy, luckily it seems upstream has a fix. I will post a debdiff for that over at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=943761 Second transitioning nototools to python 3 is only really useful if it's rbdeps fonts-noto-color-emoji and fonts-roboto-slab transition too, otherwise you are just trading one rc issue for another. Worse britney does not track build-depends properly, so packages that are only required by build-depends will be removed completely rather than kept around as cruft. A test with fonts-noto-color-emoji, showed that it was not as simple as just switching over the build-dependency, I will post a new bug report over there with more details.
