Provide some documentation for gevent.backdoor.BackdoorServer.

c-ares update

Fix Python 3 test failures. Thanks to the contributors of the following issues (at least): Fixes #412. Fixes #413. Fixes #416. Fixes #421. Fixes #462. Fixes #409.

Update the changes from #409 and merge them. Use this new FileObjectPosix to implement the socket._fileobject where possible. Fixes #409.

Better reflect the way leaktests are handled now; this should restore test__backdoor to its FLAKY status on Travis.

Python3 on certain platform (not OS X) raised 'ValueError: cant have unbuffered text I/O', so line buffer it.

Do not support patch_sys under Py3 because of differences in the interpreter that make the current approach using a threadpool unworkable. Fixes test__issue6.

Fix test__example_udp_servers.py by not subclassing _socket.socket, wrapping it instead and providing access to the wrapped socket. This is more like what Python2 does as well.

Fix test__example_echoserver.py. Text/binary issues in the server and client as well as a changed kwarg.

This handles None exception values that would otherwise fail when passed
to traceback.print_exception() and prevent the process from exiting. An associated test is also included.

Fix for issues #461 and #471.

Fix test__subprocess for Py3. A premature close of half the pipe was causing the whole pipe to get closed, apparently, at least under OS X, so don't close it. The test still fails if the fix is removed.

I'm not 100% sure this is the correct fix; could there be some underlying issue? The subprocess code is the same on both Py2 and Py3.

pywsgi mostly works, there are a few timeout issues lurking somewhere in the test suite. But it's enough for the example servers to run.

Upgraded c-ares to 1.10.0

Fix several more socket-related tests under Py3: test__examples, test__os, test__fileobject, test__subprocess_poll, test__server, test__execmodules.