1.0 - effect is now Python3-only and uses Exceptions instead of exc_info tuples

This is a major backwards-incompatible release of Effect.

Everywhere that exc_info tuples were previously used for exceptions, now we simply use Exception instances. exc_info tuples were only necessary on Python 2, because Exception objects did not carry their tracebacks. Now, on Python 3, tracebacks are embedded in the Exception instance, so we don't need to worry about that old three-tuple.

This means that Effect 1.0 only supports Python 3 and up (officially, only 3.6 and up are supported).