Python 3 support, redux

As mentioned on [the Jira ticket](https://issues.apache.org/jira/browse/THRIFT-1857?focusedCommentId=14104848&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14104848), there are still some minor problems with `fastbinary.c`, which uses a private API that no longer exists in Python 3.  I think it's worth just removing outright.

But the test suite passes completely on Python 3, now.