Set the minimum Python version to `3.8`

[misl6]

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.

[Julian-O]

#2869 covered a lot of this.