Error in trio's sys.excepthook when maximum recursion depth exceeded

[catern]

The following code triggers an error in the trio-injected sys.excepthook:

import trio

def f():
    try:
        f()
    except:
        raise Exception("foo")

f()

That is, if we "import trio" before running f(), we'll see an Error in sys.excepthook: before the main exception; if we don't import trio before running f(), the exception is printed just fine as normal, with no error in sys.excepthook.