python3 update

[王 悉奥]

Hello, I have a question about python3 package. Take the stable python3.9 as an 
example, the upstream has released to 3.9.16 which contains a lot of security 
fixes, like CVE-2022-37454 and CVE-2022-42919 in 3.9.16 and the 3.9.3 in debian 
seems kind of old and not safe. I know I can just build python3 from source, I 
just want to know why debian does not update python3 to the lastest version or 
at least apply the upstream security fixes.