fix(docs): add graphviz package to render graphviz graphs

[jules-ch]

Documentation on readthedocs fails to render graphs for numpy page https://pint.readthedocs.io/en/stable/user/numpy.html adding graphviz apt package should do the trick

[codspeed-hq]

CodSpeed Performance Report

Merging #2129 will not alter performance

_{Comparing jules-ch:fix-docs-graphviz (eb5575e) with master (c350f02)}

Summary

✅ 437 untouched benchmarks