Drop Jython

We don't actually support Jython, but we do test it in CI. All of PyMongo's tests pass with Jython 2.7.0, but not with 2.7.1, because of https://bugs.jython.org/issue2618. We keep running into more and more issues, like https://bugs.jython.org/issue2784 and https://bugs.jython.org/issue2788. It's no longer worth working around the bugs to keep the tests passing.

PyMongo 4.0 will drop all Jython specific code and stop testing Jython in CI.