Deprecate `testtools` support

At the time of writing, our documentation might give an impression that using testtools is the preferred way to write tests for a Falcon app.

I haven't done any research or survey in the area, but just subjectively it feels that the most popular testing frameworks in 2023 are pytest followed by the stdlib's unittest (both are also well supported by Falcon). Moreover, IIRC I had to exclude testtools from our test dependencies when adding Python 3.10 CI gates, since it wasn't 3.10-ready by the time. So it's becoming a maintenance burden, too.

OTOH, it seems that there has been a new testtools release, and the project is still maintained. So after all maybe it doesn't hurt to keep the integration?

Even if we decide to keep testtools, we should IMHO reword the docs not to give the impression it is the preferred way, and link to our tutorial for using pytest in the same paragraph.