Prepare 0.15.0

[phimuemue]

As agreed upon in #1085 (comment).

I adapted things from a015a68 (the 0.14.0 one).

I concur, it's a bit of work to create the CHANGELOG.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.89%. Comparing base (6814180) to head (6829485).
⚠️ Report is 184 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1091      +/-   ##
==========================================
- Coverage   94.38%   93.89%   -0.50%     
==========================================
  Files          48       51       +3     
  Lines        6665     6468     -197     
==========================================
- Hits         6291     6073     -218     
- Misses        374      395      +21     

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