Bump requests to 2.33.0 and drop Python 3.9 support

[lcbill]

Summary

Upgrades requests to 2.33.0, which patches GHSA-gc5v-m9x4-r6x2 (insecure temp-file reuse in extract_zipped_paths(); vulnerable <2.33.0).

requests 2.33.0 requires Python >=3.10, so this raises the SDK's minimum supported Python to 3.10. Python 3.9 reached end-of-life in October 2025, so this is the natural time to drop it. This supersedes Dependabot #316 (which bumped only requests and could not merge because it broke the 3.9 CI job).

Changes

• requires-python = ">=3.10"; removed the Programming Language :: Python :: 3.9 classifier.
• requests==2.32.3 -> ==2.33.0 (the security fix).
• Removed 3.9 from both CI matrices (ci.yml, publish-to-pypi.yml) and the two python:3.9 Cloud Build steps (cloudbuild_pr.yaml).
• Simplified the dependency shims that existed only for 3.9:
  • orjson is now a single uncapped >=3.10.0 (orjson 3.11+ requires 3.10+).
  • pytest pinned to 9.0.3 for all supported versions (carries the CVE-2025-71176 fix; previously the <3.10 shim held it back to 8.3.4).
  • tomli backport kept for 3.10 (tomllib is stdlib only from 3.11).
• Updated packaging tests for the new 3.10 floor and the single orjson dependency.

Test plan / validation (local, Python 3.11)

• Fresh venv pip install ".[dev]" from source resolves requests 2.33.0, orjson 3.11.9, pytest 9.0.3.
• Full suite pytest tests/unit/ tests/microbenchmarks/ --benchmark-disable: 3744 passed, 5 skipped.
• test_packaging.py: 20/20 pass (incl. updated test_requires_python_minimum and test_orjson_is_uncapped).
• python -m build produces sdist + wheel; built metadata shows Requires-Python: >=3.10, classifiers 3.10–3.14, requests==2.33.0, orjson>=3.10.0.