Set the minimum Python version to `3.8`

[misl6]

The kivy-ios project currently supports Python versions as low as 3.6.

This issue is to track the task of setting the minimum Python version to 3.8 in order to take advantage of the features and improvements introduced in later Python versions while simplifying development, CI testing, and reducing potential compatibility issues.

Action Items

• [ ] Update project documentation to reflect the new minimum Python version requirement.
• [ ] Update the project's build and testing infrastructure to enforce the minimum Python version.

[Julian-O]

Kivy-ios does not currently convey the meta-data about Python versions required to PyPi. This should be advertised there.

I think that means having a section in setup.cfg that includes classifiers, such as Programming Language :: Python :: 3.whatever for each supported version.