prevent intermittent broken_pipe errors with threaded=True

[mattwbarry]

Running my tests I am getting frequent but intermittent broken pipe errors when accessing various app endpoints. I was able to solve this locally by adding threaded=True to app.run invocation here

[vitalk]

Nice catch up, @mattwbarry! This errors mainly occur when your app trying to handle concurrent requests, for example

@app.route('/one')
def one():
    res = requests.get(url_for('two', _external=True))
    return res.text

@app.route('/two')
def two():
    return '42'

def test_should_fail(self, live_server):
    res = urlopen(url_for('one', _external=True))
    assert b'42' in res.read()