Add 'superseded by' notice to older Python 3 releases

[hugovk]

Description

Automating some more manual RM stuff from PEP 101:

• If this is a final release:
  • For 3.X.Y, edit all the previous X.Y releases’ page(s) to point to the new release. This includes the content field of the Downloads -> Releases entry for the release:

  Note: Python 3.x.(y-1) has been superseded by
  `Python 3.x.y </downloads/release/python-3xy/>`_.
  

This tedious manual work isn't often followed, and is perfect for automating over here.

So this adds a note to release pages to link to the latest one in the feature series. The latest one doesn't have the link.

For example:

• http://0.0.0.0:8000/downloads/release/python-3150a2/ no note
• http://0.0.0.0:8000/downloads/release/python-3150a1/ notes superseded by 3.15.0a2
• http://0.0.0.0:8000/downloads/release/python-3140/ no note
• http://0.0.0.0:8000/downloads/release/python-3140rc3/ notes superseded by 3.14.0
• http://0.0.0.0:8000/downloads/release/python-3139/ no note
• http://0.0.0.0:8000/downloads/release/python-3138/ notes superseded by 3.13.9
• http://0.0.0.0:8000/downloads/release/python-3137/ notes superseded by 3.13.9
• http://0.0.0.0:8000/downloads/release/python-3130/ notes superseded by 3.13.9
• and so on

I didn't bother adding it for Python 2 releases.

[captn3m0]

Love this change :)