Add 'Python 3.X.YaN' placeholder for release name

[hugovk]

Description

Add "Python 3.XY.aN" as the placeholder for the name field. Results in this HTML change:

-<input type="text" name="name" class="vTextField" maxlength="200" required="" id="id_name">
+<input type="text" name="name" class="vTextField" maxlength="200" placeholder="Python 3.X.YaN" required="" id="id_name">

This is a useful prompt, as I often have to check the existing releases to check if I need to include Python or write the short or long form of pre-release ("3.15.0a5" or "3.15.0 alpha 5").

Before: https://www.python.org/admin/downloads/release/add/

After: http://0.0.0.0:8000/admin/downloads/release/add/

And you type in the real value:

Closes

• Can be done in addition to or instead of feat: sanitize release names upon submission #2725 / Enhancement: Sanitize release names to require string #2724.