Drop Python 3.3 support

Drop Python 3.3 support

Since wheel 0.30 and later no longer works with Python 3.3, it's
increasingly hard to test. Very few old platforms are likely to lack
Python 3.4 or later so this is a relatively safe move.

Also clean up some 3.3 workarounds as they're no longer needed if we
don't support running on it.