[ci] Add Fedora 43 to the CI

[guitargeek]

Fedora 43 is not released yet, so it's marked as a special build.

The reason why it's added to the CI already early is that it already provides Python 3.14, which will be released in 3 months. So it's very useful for testing the ROOT Python interfaces with the upcoming Python version.

[dpiparo]

Thanks for this addition! We have some reports about Python 14 already, and it's useful to be able to test it.

[github-actions[bot]]

Test Results

    20 files      20 suites   3d 17h 27m 40s ⏱️  2 856 tests  2 855 ✅ 0 💤 1 ❌ 55 535 runs  55 534 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 3160bf13.

:recycle: This comment has been updated with latest results.