chore: drop unused scipy dependency

[petercorke]

scipy is listed in pyproject.toml dependencies, but pgraph never imports it and never has (git log -S scipy shows no code use).

• It first appeared in the setup.py → pyproject.toml migration (977675c). The old setup.py required numpy, matplotlib and ansitable, not scipy.
• The "powered by SciPy" README badge added in docs: restructure README top section to standard ecosystem layout #4 was based on the same mistake, so it's removed too.

Changes

• pyproject.toml: remove scipy from dependencies.
• README.md: remove the "powered by SciPy" badge.

Testing

• Installed this branch into a fresh venv and ran the full test suite: 45 passed.
• scipy is still installed there, pulled in by spatialmath-python. pgraph's own requirements are now matplotlib, numpy, spatialmath-python.

Part of #23.

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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 85.05%. Comparing base (173f548) to head (f1e1f51).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   85.05%   85.05%           
=======================================
  Files           2        2           
  Lines         776      776           
=======================================
  Hits          660      660           
  Misses        116      116           

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