cpython: bump patch versions, remove Conan 1.x code

[Ahajha]

Also removes some invalid configs:

1. No point in checking for old mpdecimal versions, since 2.4.2 (the only version before 2.5.0) is being removed.
2. GCC 9 now works on newer versions, I suspect there was an update. I tested locally with GCC 9.5.0.

I can re-add the other 3.12 versions if desired, technically it's not the latest minor version anymore.

Currently broken, waiting on #25890