Add min required python version

[Rotzbua]

Currently there is no clear statement which python versions are supported or supposed to work.
The only hint it the automated test which are

imagehash/.github/workflows/testing.yml

Line 19 in 07951cd

python-version: ['3.9.1', '3.11']

Usually there is a python_requires which defines the oldest supported (tested) python version. Most libraries sync their supported versions with the officially supported python versions.

Some more reasons from AI:

• Compatibility Check: It helps to ensure your package is installed only on compatible Python versions.
• Better User Experience: Users will be notified if their Python version is incompatible with your package, preventing installation issues.
• Automated Tools: Tools like pip will respect this requirement and prevent the installation of your package on incompatible Python versions.