chore: raise the supported Python floor from 3.8

[refeed]

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.