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'