Refactor: Deprecate Python3.9 support

[gmuloc]

Change Summary

Prepare removal of 3.9 support in 5.0.0

Component(s) name

requirements
plugins

Proposed changes

Add warning when running verify requirements

How to test

Run with python 3.9 and watch

Checklist

User Checklist

• Add to release notes in deprecation

Repository Checklist

• My code has been rebased from devel before I start
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation and documentation have been updated accordingly.
• I have updated molecule CI testing accordingly. (check the box if not applicable)

[github-actions]

Review docs on Read the Docs

To test this pull request:

# Create virtual environment for this testing below the current directory
python -m venv test-avd-pr-4177
# Activate the virtual environment
source test-avd-pr-4177/bin/activate
# Install all requirements including PyAVD
pip install "pyavd[ansible] @ git+https://github.com/gmuloc/avd.git@deprecate-python-3-9#subdirectory=python-avd" --force
# Install Ansible collection
ansible-galaxy collection install git+https://github.com/gmuloc/avd.git#/ansible_collections/arista/avd/,deprecate-python-3-9 --force
# Optional: Install AVD examples
cd test-avd-pr-4177
ansible-playbook arista.avd.install_examples

[ankudinov]

LGTM

[carlbuchmann]

LGTM