Set the minimum Python version to `3.8`

The `python-for-android` project currently supports Python versions as low as `3.7` (but somewhere still have references to `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.

Locate the project documentation and build/testing configuration that still reference Python 3.6 or 3.7. Update the stated requirement and enforcement points so Python 3.8 is the minimum, then run the relevant checks to confirm older versions are no longer supported.

Written by the indexing model from the issue text.