Improved build matrix

But I get errors running this install command; it doesn't look like there's a `py3-encodings` branch of that repo; there is, however, a `py3` branch, which seems to work for me locally.

Also, it's faster if you install from a GitHub tarball rather than by cloning the whole git repo, so this install command should be `pip install https://github.com/chtd/psycopg2cffi/tarball/py3` rather than `pip install git+https://github.com/chtd/psycopg2cffi.git@py3-encodings`