Add 1.3.15 release notes

[engechas]

Issue #, if available:

Description of changes:

CheckList:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.01%. Comparing base (2c3c74b) to head (fc23730).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.3    #1448      +/-   ##
============================================
+ Coverage     78.98%   79.01%   +0.02%     
  Complexity      268      268              
============================================
  Files           176      176              
  Lines          7287     7287              
  Branches       1017     1017              
============================================
+ Hits           5756     5758       +2     
+ Misses         1004     1003       -1     
+ Partials        527      526       -1     

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