Fix broken tests. Drop python 3.8.

[rectalogic]

A number of tests are failing on main, apparently because tests always use the latest version of every dependency (no Pipfile.lock) and e.g. pytest_httpx has breaking changes.

Also pytest_httpx now requires python>=3.9 so I dropped 3.8 support.

What do you think about switching from setup.py to isolated builds using uv and checking in a uv.lock so tests don't fail over time?

=========================== short test summary info ============================
FAILED tests/test_keys.py::test_uses_correct_key - assert 1 == 0
FAILED tests/test_templates.py::test_template_basic[boo-extra_args3-None-None-Error: Cannot use -t/--template and --system together] - TypeError: HTTPXMock.reset() got an unexpected keyword argument 'assert_all...
FAILED tests/test_templates.py::test_template_basic[prompt: 'Say $hello'-extra_args4-None-None-Error: Missing variables: hello] - TypeError: HTTPXMock.reset() got an unexpected keyword argument 'assert_all...
ERROR tests/test_keys.py::test_uses_correct_key - AssertionError: The following requests were not expected:
ERROR tests/test_templates.py::test_template_basic[boo-extra_args3-None-None-Error: Cannot use -t/--template and --system together] - AssertionError: The following responses are mocked but not requested:
ERROR tests/test_templates.py::test_template_basic[prompt: 'Say $hello'-extra_args4-None-None-Error: Missing variables: hello] - AssertionError: The following responses are mocked but not requested:
============ 3 failed, 181 passed, 638 warnings, 3 errors in 3.23s =============
error: Recipe `test` failed on line 10 with exit code 1

[simonw]

Thanks for this - yeah, I've been meaning to port to pyproject.toml and I'd be pleased to get uv working too.