Refactor the Travis build to make the matrix explicit and faster, simplify Python installs 

Right now the different versions aren't getting combined.

It's not clear if this will work or not.

This should eliminate cache thrashing when we upgrade python versions. And also eventually might do more caching here.

Also simplify the paths at which pythons are installed, so we only have to change one place to update a version.

The cache is only shared between stages that match things like environment variables.

Ah, we need CFFI at runtime; build isolation means pyproject.toml doesn't deliver it to us. We must have had it cached.

A DNSPython test we didn't run before sometimes fails. Must be multiple answers.

Tweak test coverage and test_ssl for PyPy3

Refactoring the testrunner; more needed, but this gets us simple travis folding.

Reorganize the test matrix for clarity, and shrink it back to what we used to test.

Remove now-unused entries from the makefile.

Remove the makefile, port the posix specific rules to Python for 'setup.py clean'

Update to libev 4.25

Test libuv and c-ares in non-embedded cases

[skip appveyor]

Fixes #1395

Actually, I think maybe not all backends will work. Lets find out.

[skip appveyor]

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