Require Python >= 3.9

Python 3.8 is official EOL.

[EOL Announcement](https://discuss.python.org/t/python-3-8-is-now-officially-eol/66983) | [Status of Python versions](https://devguide.python.org/versions/)

The next txtai release will require Python 3.9. The build scripts, docker images and anything else with 3.8 support should be updated. 

