Fix toml-test scripts (#433)
Update tests/decoding_test.py to use the format used by the current
version of toml-test. I also removed Python 2 support (my distro doesn't
ship it, so I can't test it).
Also add tests/encoding_test.py for encoder tests
Still quite a few failures, mostly because it looks like it hasn't been
updated to use 1.0 yet:
% toml-test ./decoding_test.py | tail -n2
valid tests: 139 passed, 18 failed
invalid tests: 230 passed, 53 failed
% toml-test ./encoding_test.py -encoder | tail -n1
encoder tests: 146 passed, 11 failed5 files changed
