update CI matrix for supported Python versions

[jGaboardi]

• drop
  • 3.8
  • 3.9
• add
  • 3.12

---

• update
  • pyproject.toml
  • testing.yml
  • build_docs.yml
  • docs/installation.rst

---

xref

• #299 (comment)
• #309

[jGaboardi]

closed via #318