[build] Signal explicitly to users the PyPI distribution is in alpha

[vepadulano]

By injecting a warning in the init.py file of the package before starting the build of the wheel.

This change is added to make the situation of the Python wheel distribution as clear as possible and to manage user expectations, in line with the discussions held during the ROOT Users Workshop 2025.

See some examples of questions and feedback already existing which could have benefited from a more clear description: #20472
https://root-forum.cern.ch/t/is-this-a-legitimate-release-of-root/64429

[vepadulano]

An example of the warning when installing the wheel (e.g. on lxplus)

(myenv) [vpadulan@lxplus9101 /tmp/vpadulan]$PYTHONPATH=/tmp/vpadulan/myenv/lib/python3.13/site-packages/:$PYTHONPATH python
Python 3.13.9 (main, Nov  6 2025, 00:07:41) [GCC 13.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ROOT
/tmp/vpadulan/myenv/lib/python3.13/site-packages/ROOT/__init__.py:5: UserWarning: 
This distribution of ROOT is in alpha stage. Feedback is welcome and appreciated. Feel free to reach out to the user forum for questions and general feedback at https://root-forum.cern.ch or to submit an issue at https://github.com/root-project/root/issues. Do not rely on this distribution for production purposes.

[github-actions]

Test Results

    22 files      22 suites   3d 17h 22m 22s ⏱️
 3 776 tests  3 776 ✅ 0 💤 0 ❌
81 164 runs  81 164 ✅ 0 💤 0 ❌

Results for commit 16aeb00.