Use -Wdefault when running Python testdriver (#22727)

This enables Python's -Wdefault when running the testsuite driver, to catch deprecated code.

This has caught several string literals using invalid escape sequences, which I had mostly fixed in !9669 (closed) but forgot about one.

In runtests.py I had to change open(arg) to io.open(arg), otherwise mypy fails. This is strange, since in Python 3 open and io.open are supposed to be the same. This seems to be a bug that has been fixed; if I run mypy locally, it does not give this error (it gives different failures, but that can be fixed later).

I removed sys.stdout = os.fdopen(sys.__stdout__.fileno(), "w"). I wrote that code originally when porting to Python 3 for symmetry with Python 2 but I believe it was never needed.

Fixes #22727 (closed).