Update supported Python versions

[jhkennedy]

This:

• Drops Python 3.7 and 3.8 support by requiring Python >=3.9
• Explicitly states support for Python 3.11 and 3.12
• removes unecessay importlib_metada package as it's now included in the standard library (Python 3.8+) as importlib.metadata

Also:

• drops the install warning that's no longer needed now that we've moved to a src directory.