However upstream python-keyring has dropped Python 2 support, and I want
to upgrade to a newer release, so Python 2 support will be dropped sooner
or later.
upstream drop of Python 2 support doesn't mean you need to drop it too. For example I'm keeping pytest at lower version for same reason. Please respect Python 2 removal and release team rules and don't remove Python 2 support untill all reverse(-build) depends (+autopkgtests) are fixed. Removing of python-keyring is blocked by 4 bugs, But recursively it's dozens, for example python-mock is affected. Removing this means marking dozens off Debian packages as AUTORM. Please be careful.
If you really need to upgrade python-keyring to newer version, you could split it into two source packages. But do it only if it's really needed.
Thanks for understanding.