Avoid a memory leak when wrapping an io.BufferedWriter around a socket.

Implement the fix in Cython so we don't lose any speed (we actually benchmark a bit faster).

Fixes #1318 and fixes #1319

 Patch the 'thread' module provided by 'future' if it's already imported

Patch the 'thread' module provided by 'future' if it's already imported, which is likely because pkg_resources imports it if installed.

Also some more cleanup from removing py3.4 support.

Took some tweaking of connection params --- did something change in OS X? And PyPy's GC has apparently changed the way it deals with sockets, we're better off being fully explicit about lifetime.

Fixes #1354. Fixes #1357

- Remove 3.4
- Update to latest 3.x series
- Update to PyPy 7

Tests are not passing locally yet.

Fix a typo in doc/intro.rst

Remove unused

Remove useless variable

Upgrade libuv to 1.24.0

More tweaks for test__fileobject.py on 3.7

Deterministically close GreenFileDescriptorIO backing FileObjectPosix if the (libuv) event loop watchers produce an error.

Especially on Python 3.7, the C-level garbage collection could kick in and close the underlying fileno at an inopportune time, killing an unrelated file, even though there were no Python-level references alive (apparently).

Cast OSError raised by os.lseek into IOError