Why can't I install python3.8-dev on Ubuntu 16.04?

Question

This comes as a wider issue of Flask with Apache 2.4 throwing a hissy fit over which version of Python they want to use, but I can't follow any of the guidance I've found to fix it, because I need python3.8-dev and it doesn't want to install:

apt install python3.8-dev
...
E: Unable to locate package python3.8-dev
E: Couldn't find any package by glob 'python3.8-dev'
E: Couldn't find any package by regex 'python3.8-dev'

I have already done:

add-apt-repository ppa:deadsnakes/ppa
apt install software-properties-common
apt update
apt upgrade

which all succeed. However, none have so far led to success with installing python3.8-dev.

The output of lsb_release -a is Ubuntu 16.04 (Xenial Xerus):

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.7 LTS
Release:        16.04
Codename:       xenial

I feel like I'm going mad. Everything worked fine and today it just didn't. I do of course already have Python 3.8 installed.

Answer 1

ppa:deadsnakes was decommissioned for Ubuntu 16.04 (Xenial Xerus). Try to build a custom .deb package.