Update dependencies + drop Python 3.8

[cdce8p]

This would be great to have for Python 3.14 in Home Assistant.

• Update all dependencies to unblock poetry updates.
• Remove upper version caps for click, requests and rich. For libraries it's generally recommended to only pin the lower bound. Especially simple libs are often times fully compatible with a new major release. Having an upper bound prevents downstream applications from updating even and would require (unnecessary) work.
• Most test dependencies have dropped Python 3.8 by now as it has been unsupported for almost a year now. Update the lower Python bound to ^3.9.
• Remove async-timeout. Though it was imported, all of them were unused. Also cleaned up additional unused imports.
• Minor typing and code style improvements (some suggested by pylint).
• Used black to update formatting.