[3.8] venv: Update Aspen URL in 'activate' script comment (GH-21797)

[3.8] venv: Update Aspen URL in 'activate' script comment (GH-21797)

A comment in the venv `activate` script (as well as `activate.csh` and `activate.fish`) referencing Aspen magic directories lists a "further information" URL for Aspen at the zetadev.com website. zetadev.com changed ownership in 2019, and now redirects to a server in China with an expired security certificate.

Out of an abundance of caution, while not changing the _code_ for the activate scripts, this PR updates the URL in those comments to reference Aspen's new documentation home at https://aspen.io/.

No issue created, as I suspect this falls within the definition of a "trivial" change. Please let me know if I'm wrong about that, and I'll open the necessary issue(s).

While filed against the 3.8 branch, strictly speaking this is not a backported PR. The comment in question was entirely removed from the script between Python 3.8 and 3.9.

(IMHO this _should_ probably be backported to 3.7 and 3.6, as well. I'll happily file those PRs if needed.)

Automerge-Triggered-By: @vsajip