Using six.raise_from wherever possible.

[dhermes]

I'm not entirely sure how worthwhile this is, I just discovered the implementation doesn't do anything helpful in Python 2 (e.g. print the traceback from the original exc):

# Python < 3.2
def raise_from(value, from_value):
    raise value
# Python == 3.2
def raise_from(value, from_value):
    try:
        if from_value is None:
            raise value
        raise value from from_value
    finally:
        value = None
# Python > 3.2
def raise_from(value, from_value):
    try:
        raise value from from_value
    finally:
        value = None

I was "inspired" to do this by #211.

[dhermes]

@jonparrott I updated so that the wrapped exceptions were still wrapped.