Clean up `.gitignore`

[edmorley]

• Add .DS_Store (macOS metadata files)
• Add .venv/ since it's a commonly used virtual environment directory name (the existing venv/ name is still ignored too)
• Switch from *.pyc to __pycache__/ (the former is the Python 2 style)
• The redundant getting-started/* entry has been removed (I presume this might have been a leftover from the "install local package as egg" days

GUS-W-13322128.