Prepare 5.1.9

[phil-davis]

Releasing this as 5.1.9 will allow projects that are using sabre/dav 4.7.0 and 4.7.1 to pick up this event code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.45%. Comparing base (5cd7d4d) to head (56aa0f8).
⚠️ Report is 3 commits behind head on 5.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.1     #170   +/-   ##
=========================================
  Coverage     99.45%   99.45%           
  Complexity      119      119           
=========================================
  Files             7        7           
  Lines           366      366           
=========================================
  Hits            364      364           
  Misses            2        2           

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