stubtest_third_party.py, mypy_test.py: just print everything to stdout

[AlexWaygood]

In GitHub Actions, all stderr appears above stdout, even if things are printed to stdout chronologically earlier than being printed to stderr. That is, if you have something like

print("foo")
print("bar", file=sys.stderr)

The output in GitHub Actions will be

bar
foo

Recent changes I made to stubtest_third_party.py made the output go funny because some things were being printed to stderr in CI and some things to stdout. I fixed most of that in #11799, but not all of it (the output still isn't quite right in https://github.com/python/typeshed/actions/runs/8776498752), and now the output of mypy_test.py is all screwed up because of that fix (https://github.com/python/typeshed/actions/runs/8782468512/job/24096628717?pr=11808).

I think it's easier if we just print everything to stdout rather than passing file=sys.stderr anywhere, because if you forget to do it in one place then you end up with bizarre output in CI. Ideally some of this information would be going to stderr rather than stdout, but I don't think it matters that much for our purposes; mostly our scripts are just keeping us up to date with where they're at.