retry get price tiers which seems to fail intermittently (bug 1024065)

it would be great to add one more test where the API call to prices raises a connection error *once* but returns successful the second time. You could do it something like:

    try_count = {'count': 1}

    def prices():
        if try_count['count'] > 1:
            return '<real price mock'>
        else:
            try_count['count'] += 1
            raise ConnectionError()
        
    slumber.webpay.prices.side_effect = prices