Update to libuv 1.27.0

Require dnspython 1.16 and backport a fix from 2.0

This lets us have a much simpler integration.

Fixes #1258

Add pyproject.toml

Use test.support.find_unused_port() instead of rolling our own; it handles windows better.
Limit the DNS6 tests to known good configurations.
Hopefully fix a bunch of flakiness in test__refcount's use of sockets.

Fixes #1389

Tests do reasonable things when some of the test deps are missing

Make the optional backends optional in test__all__ and test__execmodules

Fixes #1366.

Fix minor typo in docstring

Performance and correctness improvements for Greenlets.

- Rework getting the stack, since we're no longer arbitrarily extending it. This is faster, and I found and fixed a bug on CPython.
- Let Cython do more of the work making sure we really have a hub as our parent.

Update process.py example to work on windows.

Upgrade to PyPy3.6-7.1. Fixes #1373.

The "Unable to run on windows" warning is actually about uname/ls, not
about gevent :)

libev-cffi: Use nlink_t and let the compiler fill in the definition.