feat: Drop Python 3.8 support and add Python 3.13 tests

This commit drops support for Python 3.8, which was failing due to an
issue with an older version of setuptools and the license field in
pyproject.toml.

The supported Python version range is now '>=3.9'.

Additionally, this commit adds a test job for Python 3.13 to the CI
workflow to ensure compatibility with the latest Python version.

fix: Update indices in noxfile.py

This commit updates the indices in noxfile.py to point to the correct
python versions after the list of supported versions was changed.

Author: google-labs-jules[bot]

noxfile.py

@@ -106,7 +106,7 @@ def activate_virtualenv_in_precommit_hooks(session: Session) -> None:
                 break
 
 
-@session(name="pre-commit", python=python_versions[2])
+@session(name="pre-commit", python=python_versions[1])
 def precommit(session: Session) -> None:
     """Lint using pre-commit."""
     args = session.posargs or [
@@ -133,7 +133,7 @@ def precommit(session: Session) -> None:
         activate_virtualenv_in_precommit_hooks(session)
 
 
-@session(python=python_versions[2])
+@session(python=python_versions[1])
 def safety(session: Session) -> None:
     """Scan dependencies for insecure packages."""
     requirements = session.run(
@@ -196,7 +196,7 @@ def tests(session: Session) -> None:
             session.notify("coverage", posargs=[])
 
 
-@session(python=python_versions[2])
+@session(python=python_versions[1])
 def coverage(session: Session) -> None:
     """Produce the coverage report."""
     args = session.posargs or ["report", "-i"]
@@ -209,7 +209,7 @@ def coverage(session: Session) -> None:
     session.run("coverage", *args)
 
 
-@session(python=python_versions[2])
+@session(python=python_versions[1])
 def typeguard(session: Session) -> None:
     """Runtime type checking using Typeguard."""
     session.install(".")
@@ -232,7 +232,7 @@ def xdoctest(session: Session) -> None:
     session.run("python", "-m", "xdoctest", *args)
 
 
-@session(name="docs-build", python=python_versions[5])
+@session(name="docs-build", python=python_versions[4])
 def docs_build(session: Session) -> None:
     """Build the documentation."""
     args = session.posargs or ["docs", "docs/_build"]
@@ -258,7 +258,7 @@ def docs_build(session: Session) -> None:
     session.run("sphinx-build", *args)
 
 
-@session(python=python_versions[2])
+@session(python=python_versions[1])
 def docs(session: Session) -> None:
     """Build and serve the documentation with live reloading on file changes."""
     args = session.posargs or ["--open-browser", "docs", "docs/_build"]