Fix plum 2.x import and release test config

[astanziola]

• Fix plum.resolver.NotFoundLookupError import (removed in plum-dispatch 2.x)
• Clear pytest addopts in release workflow to avoid nbval dependency
• Changelog entry added under 0.3.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (2d379c3) to head (4f37d62).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #170   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files          15       15           
  Lines         947      947           
=======================================
  Hits          885      885           
  Misses         62       62           

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