Deprecate the old EMBED vars in favor of GEVENTSETUP_EMBED.

Refs #1402

Also documented the extras, and added a simpler one.

Produce a better environment string for USE_RESOURCES, and actually pass the right version to the environment variable so we're not just adding on.

With the  exception of the generic examples; those might need reviewed still.

I found places to support the 'network' and 'subprocess' resources.

The bundled tests use the network resource; interestingly, they don't use the subprocess resource.

The testrunner now has an argument for resources, and sets those up based on that or the environment variable.

Using the standard library resource names. Currently this must be set
with the environment variable GEVENTTEST_USE_RESOURCES (e.g.,
GEVENTTEST_USE_RESOURCES=all,-network to disable stdlib network
tests).

Still to do:

- Documents and change note
- Command line argument in testrunner
- Enable from gevent tests

First part of fixing #1349: Allow test.support.is_resource_enabled to do its usual thing in monkey-patched tests (and eventually our own tests).

By default, this will always return true, so nothing will be skipped. An eventual step is to add options to the testrunner to control which resources are available, analogous to what test.regrtest can do.

Because we always spawn fresh subprocesses, we'll have to do this with environment variables and be sure we have the right setup code always invoked.

Tweak appveyor for now and add instructions to workaround pip 19.1

[skip travis]

The RTD pseudo-sidebar that we get now (injected from a script?) messes up the page layout. Remove it.
[skip ci]

Fix Sphinx 2.0 warnings.

Inherit the default search.html template since ours was broken. Visibly it's pretty much the same, but this one works.

Fix a CSS error that made the search boxes look really bad.

Tested on 1.8 with Python 2.7 and 2.x with Python 3.7. Requires 1.8

[skip ci]

[skip ci]

Update c-ares to 1.15

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