Drop Python 3.9 support

[Copilot]

Python 3.9 support removed to align with updated dependency requirements (urllib3 2.6.3+).

Changes:

• Bump minimum Python version from 3.9 to 3.10 in pyproject.toml and docs/pyproject.toml
• Remove Python 3.9 from package classifiers
• Update mypy target from 3.9 to 3.10
• Remove Python 3.9 from CI test matrix

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