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.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.79%. Comparing base (3376eb5) to head (94c7331).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   94.79%   94.79%           
=======================================
  Files           3        3           
  Lines        1249     1249           
=======================================
  Hits         1184     1184           
  Misses         65       65           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.