1.2.3

Despite the less-than-minor version bump, under the hood there are some very notable changes.

Some highlights:

• Simplified MANIFEST.in -- this should result in a much smaller package
• Stop using Appveyor and Travis CI, fully using GitHub Actions now for testing
• Lots of improvements in documentation
• Now uses pytest for the test runner instead of nose2
• Lots of improvements in tox.ini
• Some behavior fixing:
  • Catch exceptions during controller.start()
  • More correct SMTP Status Codes
  • Correct handling of long lines and/or oversized DATA
  • Correct handling of user-supplied TLS Context
• Now implements optional command call limit