Add missing dependency group for async SQLAlchemy tests

[Dev-iL]

Changes

This PR contains two fixes:

1. Fix for a CI issue:

FAILED tests/plugins/test_polars_extensions.py::test_polars_database - ImportError: The SQLAlchemy asyncio
module requires that the Python 'greenlet' library is installed.  In order to ensure this dependency is
available, use the 'sqlalchemy[asyncio]' install target:  'pip install sqlalchemy[asyncio]'

2. Fix for Tkinter (matplotlib's default backend) not being detected when running tests locally in a uv venv.

How I tested this

Ran the below locally and verified tests complete successfully.

  uv sync --group test
  uv pip install "kaleido<0.4.0"
  uv run pytest tests/ --cov=hamilton --ignore tests/integrations

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.