add f-string parsing for py3.6+

[zhudotexe]

Includes requested changes from #37.

[coveralls]

Coverage decreased (-0.1%) to 99.853% when pulling 0698095 on mommothazaz123:fstrings into 1c29dac on danthedeckie:master.

[coveralls]

Coverage decreased (-0.1%) to 99.853% when pulling 0698095 on mommothazaz123:fstrings into 1c29dac on danthedeckie:master.

[danthedeckie]

Looking much better.
However, at +441:

def _eval_joinedstr(self, node):
    evaluated_values = [str(self._eval(n)) for n in node.values]
    if sum(len(v) for v in evaluated_values) > MAX_STRING_LENGTH:
        raise IterableTooLong("Sorry, I will not evaluate something this long.")
    return ''.join(evaluated_values)

Does this evaluate all the nodes anyway, even if they were going to produce a super long string, and simply not join them? Is there a way to safely (but not too slowly...) mitigate this? Or am I being overly paranoid?

[zhudotexe]

Hm, I don't know what I was thinking writing that. This should immediately halt evaluation if the resulting string grows too large; the recursive call should raise the same error if any one individual node exceeds the limit.