drop official python 3.5 support

Python 3.5 has reached end of life and python modules are dropping support requiring python >= 3.6 which makes supporting following distros shipping python 3.5 a nightmare:

• Ubuntu 16.04 Xenial Xerus (EOL April 2021)
• Debian 9 oldstable (LTS EOL June 30, 2022)

I've worked hard to provide python 3.5 support in apkg in order to support older distros but in reality it was a mistake to burn so many cycles on trying to do that and docker-based CI isn't making it any easier.

It's impossible to implement #41 (closed) cleanly as python3 -m build simply doesn't work on python 3.5 and my changes are being held back by artificial CI issues specific to python 3.5 in a docker container.

Current python 3.5 compat features will remain in apkg until Debian 9 reaches EOL but python 3.5 CI will be disabled for now and possibly re-enabled as optional-only without self tests (that require working python3 -m build) during #24 (closed) to provide unofficial support only on a best effort basis.

Source packages for older distros can still be created using apkg on a supported platform.