GAE/python site is no longer handling requests prefaced with 'www.'
Here's how I remedied this:

I went to console.developers.google.com > project > hockeybias-hrd > appengine > settings > domains > add
In 'step 2' on that page I put the 'www' for the subdomain in the textbox which enabled the Add button.
I clicked on the 'Add' button and the issue was solved.

I will note that this was the second time I have been 'head-faked' by google's use of greyed-out text to mean something other than 'disabled'... 'www' was the default value in the subdomain textbox - BUT it was greyed-out AND the 'Add'button was disabled right next to it. So, I did not initially think I could enter a value there.
If a user enters ‘hockeybias.com’ into his/her browser as a URL to get to my hockey news aggregation site, the default page comes up correctly. It has in the past and does so today.
However, as of this summer, if someone uses ‘www.hockeybias.com’ the user will get the following error message:

Error: Not Found
The requested URL / was not found on this server.
This is a relatively new issue for me as ‘www.hockeybias.com’ worked fine in the past.
The issue seems to have come up after I migrated from the ‘Master/Slave Datastore’ version of GAE to the ‘High Replication Datastore’ (HRD) version of GAE earlier this summer.
The issue occurred while the site used python2.5. And I migrated the site to python2.7 this morning and am still having the issue.