More automation: update README for minor version bump

[hugovk]

We're only bumping the first line ("This is Python version 3.15.0 alpha 0") and for all releases (in tweak_readme()).

There's some other 3.14->3.15 things we can automate for the first beta, see python/cpython#134649.

The PEP number is one thing that will need doing manually, we can add it to the list of manual files in bump(), but it would be good to print what needs doing, because otherwise it's only obvious to look for 3.14->3.15 things.

[corneliusroemer]

Also this should be printed/automated:

• Python 3.15 is now main devguide#1582