VersionInfo.next_version() return type is uncertain because at line 463 of semver.py return version.replace(prerelease=None, build=None) returns a VersionInfo object, whereas in line 466 it is explicitly converted to str and at line 470, version.bump_prerelease() is documented with rtype: str.
This is problematic because if you call this on a version that has a prerelease or build number, you get a VersionInfo object, and otherwise you get a str, and these two types must be handled quite differently.

Personally, I would expect VersionInfo.next_version() to return a VersionInfo object in all cases, but I think the main problem here is the ambiguity of the return type.