Add a GitHub Action to lint Python code with ruff

[cclauss]

• https://docs.astral.sh/ruff
  • https://github.com/astral-sh/ruff-action

Test results: https://github.com/cclauss/Mock.GPIO/actions

% ruff check

Error: Mock/GPIO.py:5:8: F401 `time` imported but unused
Error: Mock/GPIO.py:83:9: F841 Local variable `setModeDone` is assigned to but never used
Error: examples/test.py:3:1: E722 Do not use bare `except`

Fix ruff rule F841 issue using the same changes proposed in:

• Fix bug with setmode not setting variables correctly #16