fix(python): fix aiohttp test and revert timeout type for mypy compat

- Move `import seed._default_clients` before `mock.patch.dict` so the
  module stays in sys.modules after the context manager restores state,
  fixing the ImportError on cleanup reload
- Revert _make_default_async_client timeout param to Optional[float]
  to satisfy mypy (the ternary _defaulted_timeout can't be narrowed)
- Update poetry.lock for new aiohttp optional dep

Author: jsklan