borgmatic 1.5.13

• #373: Document that passphrase is used for Borg keyfile encryption, not just repokey encryption.
• #404: Add support for ruamel.yaml 0.17.x YAML parsing library.
• Update systemd service example to return a permission error when a system call isn't permitted
  (instead of terminating borgmatic outright).
• Drop support for Python 3.5, which has been end-of-lifed.
• Add support for Python 3.9.
• Update versions of test dependencies (test_requirements.txt and test containers).
• Only support black code formatter on Python 3.8+. New black dependencies make installation
  difficult on older versions of Python.
• Replace "improve this documentation" form with link to support and ticket tracker.