chore: raise the supported Python floor from 3.8

setup.py still declares python_requires=">=3.8" and pyproject.toml targets py38, but
Python 3.8 reached end of life in October 2024 (3.9 followed in October 2025). The floor should
move to at least 3.10, ideally 3.11.

Tasks

• Bump python_requires and the trove classifiers in setup.py.
• Bump python_version in the Pipfile and target-version in pyproject.toml.
• Update the CI test matrix to the supported versions.
• Remove version shims that become dead — e.g. dependency markers like python_version >= "3.9"
  in setup.py extras can become unconditional.
• Verify all pinned dependencies support the new floor.

Start by auditing the Python version declarations and dependency markers in setup.py, Pipfile, and pyproject.toml, then locate the CI test matrix. Update the supported floor, classifiers, matrix, and obsolete shims; verify pinned dependencies support it and run the project’s CI tests. Done means all configuration consistently supports the chosen Python versions.