Log Message:
Update to 4.1.0. From the git log:

- apparently the Let's Encrypt test server is rejecting example.com emails
- added official python 3.7 support in setup.py
- fixed #226, start using POST-as-GET for GET requests
- fixed additional POST-as-GET
- addresses #205, a situation where polling may hang indefinitely
- Also accept critical SAN extensions.
- fixed #222, we shouldn't delete the challenge files on errors, but we
  should clean them up on success