CHANGELOG
* Support POST-as-GET from updated ACME spec (RFC 8555) - Let's Encrypt will drop support for plain GET requests in November 2019 (issue #226)
* Added explicit Python 3.7 support and fixed continuous integration tests
* Added timeout to request polling to prevent infinitely waiting (pull #205)
* Added support for critical SAN extensions (issue #216, pull #217)
* Started cleaning up challenge files on success (pointed out by @felixfontein in issue #222)