feat(deps)!: Migrate from httpx to httpx2 and drop Python 3.9

[fornwall]

Replaces the httpx[http2] == 0.28.1 dependency with httpx2[http2] == 2.13.1, Pydantic's maintained fork of httpx.

Its only API change is the package rename (httpx → httpx2, httpcore → httpcore2), so the code change is mechanical: imports and httpx.X references in _http_client.py, _retry.py, _utils.py, messaging.py and the tests. Internal helper names such as handle_platform_error_from_httpx are unchanged.

Breaking: Python 3.9 dropped

httpx2 requires Python >= 3.10 (it dropped 3.9 in 2.1.0), so:

• setup.py: python_requires='>=3.10', version guard and classifiers updated
• CI matrix: 3.10–3.13 + pypy3.10 (was 3.9–3.13 + pypy3.9); lint, release and nightly jobs now use 3.10
• README / CONTRIBUTING updated

Is this ok to do now, considering that python 3.9 went EOL at 2025-10?

Test setup: respx

respx only patches the httpx module (and still pulls it in as a dependency), so tests/test_retry.py would otherwise make unmocked requests. tests/conftest.py calls httpx2.alias_httpx() so import httpx resolves to httpx2 and respx patches the classes the SDK uses. The respx pytest plugin is disabled via addopts = -p no:respx in setup.cfg, since it imports httpx at startup before conftest can alias it; no test uses its respx_mock fixture.

Other behaviour changes from httpx2

• User-Agent changes from python-httpx/… to python-httpx2/…; the logger is now named httpx2
• TLS verification uses the OS trust store via truststore instead of certifi

AI disclaimer: Created with gpt-6 astra using codex. Iterated on and reviewed by me.

[gemini-code-assist]

Code Review

This pull request bumps the minimum supported Python version to 3.10 and migrates the HTTP client library from httpx to httpx2 across the codebase, including the core HTTP client, retry logic, messaging module, and test suite. To support this migration, a pytest configuration was added to alias httpx2 before respx is imported. The review feedback suggests avoiding pinning the httpx2 dependency to an exact version in setup.py to prevent dependency conflicts for downstream applications, recommending a compatible range instead.