Add classifier and testing for Python 3.12

### pynetbox version

v7.3.0

### NetBox version

v3.7.0

### Feature type

Change to existing functionality

### Proposed functionality

Add Python 3.12 in setup.py classifiers and in the test runs.

### Use case

Python 3.12 has been out for three months, and the first fix release 3.12.1 is also out already, so let's test against it as well.

### External dependencies

_No response_