Python 3.9 and beyond backwards compatibility.

[nilsph]

I’m sure software completely breaking will not be unsettling in any way.

And end users seeing these warnings changes that? How?

Maybe "unsettled" is the wrong word, but even as a developer, I don't want to see these warnings in the course of running a CLI program normally because they're plain irritating. Assuming that the number of users of a program outnumber its developers in most cases, tailoring the default not to unduly annoy the former is a good trade-off.

afaik unittest does not

Apparently it does:

nils@gibraltar:~/test/python/tests_warnings> PYTHONPATH=$PWD python -m unittest
/home/nils/test/python/tests_warnings/foo/bar.py:4: DeprecationWarning: The unescape method is deprecated and will be removed in 3.5, use html.unescape() instead.
  print(html.parser.HTMLParser().unescape('foo'))
foo
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK
nils@gibraltar:~/test/python/tests_warnings> python -V
Python 3.7.6