Add Python 3.14 support, drop EOL 3.8

[elprans]

No description provided.

[ddanier]

As of no pre-built package being available for asyncpg on Python 3.14 you cannot install the package on "slim" docker images, cause gcc is not installed there. This is an issue as you tend to use those "slim" variants when you care about security. Of course it is possible to get this working by either compiling this package yourself on a non-"slim" image and then use a multistage build or switching to the non-"slim" version, but that either does increase complexity or reduce security significantly.

What I want to say: I really would like this to get finished and merged soon. Thanks for all the effort! ;-)