Update versions in README.rst

[hugovk]

Fixes #251.

Right now, tweak_readme() updates:

This is Python version 3.14.0 alpha 3
=====================================

This PR also updates these bits:

We have a comprehensive overview of the changes in the `What's New in Python
3.14 <https://docs.python.org/3.14/whatsnew/3.14.html>`_ document.  For a more

`Documentation for Python 3.14 <https://docs.python.org/3.14/>`_ is online,

See `PEP 745 <https://peps.python.org/pep-0745/>`__ for Python 3.14 release details.

We also have a sentence like this, but I don't think we need to bother updating it each time:

For example, if you want to install Python 2.7, 3.6, and 3.14 with 3.14 being the
primary version, you would execute ``make install`` in your 3.14 build directory
and ``make altinstall`` in the others.