chore: drop support for Python 3.9

[phanak-sap]

Python 3.9 had EOL in 2025-10-31, this had enough delay move to 3.10 as minimal version (even considering that in october 3.10 version will reach its own EOL - pyodata usually drop support later than lang itself.)

tasklist:

• Remove "3.9" from CI python-version list, drop lxml 4.3.5/4.4.3/4.5.2 from the matrix, and remove the
  now-redundant Python 3.10 excludes for those versions
• Remove Python :: 3.9 classifier and add python_requires='>=3.10' to setup.py
• Update Python version requirement from 3.9 to 3.10 in README.md
• Add Python 3.9 deprecation entry to CHANGELOG.md [Unreleased] section
• Replace deprecated typing.ByteString with bytes in tests/test_vendor_sap.py