changelog: the sdist no longer ships build/lib or RELEASE

[petercorke]

The release notes did not mention the two leftovers removed from #14 just before merging: build/lib/pgraph/*.py, a stale old copy of the source that was in the sdist next to the real one, and RELEASE, a MATLAB-era file that held 0.6.1. The sdist contents are user-visible, so they belong in the entry.

Documentation only.

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[codacy-production]

Up to standards ✅

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (3947a02) to head (8fbb1d5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files           4        4           
  Lines        1586     1586           
=======================================
  Hits         1547     1547           
  Misses         39       39           

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