Infra: Include trailing slash in RSS URLs to avoid 301 redirect

[hugovk]

https://peps.python.org/peps.rss includes links like:

      <guid isPermaLink="true">https://peps.python.org/pep-0686</guid>

Without the trailing /, there's an extra HTTP 301 redirect to add the slash, which adds a small but unnecessary delay which can vary between 20ms and 335ms.

See sphinx-doc/sphinx#10288 for some network waterfall diagrams with and without the extra request.

So let's include it:

      <guid isPermaLink="true">https://peps.python.org/pep-0686/</guid>

Also update pip and include wheel in virtualenv creation to remove a couple of warnings, noticeable during make rss.

[CAM-Gerlach]

LGTM @hugovk