Fix TypeError in render_exception() on Python 3 (#21)

Also fix TypeError in trim_docstring on Python 3, when --pypi-strict is
enabled -- the tests I had exposed both bugs, once I fixed the tests to
test real-world.

These bugs are a consequence of #16: trying to be nice by accepting
invalid input (i.e. input that doesn't match your locale charset) means
we deal with bytestring internally in places that before had proper
Unicode strings.