While the contribution to Python Cryptography is incredibly awesome, I'd suggest that perhaps the lede is slightly buried. Having a large suite of well documented test vectors, including both positive and negative examples, raises the bar for the entire ecosystem. Go and OpenSSL are mentioned, but any other environment implementing X.509 now has another very well developed test suite that can be plugged in directly.
While I'm a huge fan of formal methods for developing secure code, IMHO well developed suites such as x509-limbo[1] with common, good, problematic, and outright sadistically wrong vectors are the way forward for developing robust cross platform protocols. These are the kinds of unit tests a spec should come with.