I've built a Windows version of Python 3.9.x and successfully built Sigil with it (I've also used a stock Python.org 3.9.x successfully), but I've not tested it all that heavily for any subtle problems. But sans any huge issues that I might encounter, I've no problem with moving to 3.9.8. The main thing I have to keep in mind for Windows is PyQt/Qt/Python/Python module compatibility. That "magic" intersection where everything will play nice together.
I just have to avoid making Python 3.9.8 any kind of hard minimum, so that I don't leave a bunch of *nix distros in the dust.