Fix missing import, wrong ApiException path, and CI/CD issues

[anthonylouismorton]

Changes

• Add missing import os in README Getting Started example — without it the example raises NameError when copy-pasted
• Fix ApiException import to use cfbd.exceptions instead of cfbd.rest (canonical source)
• Fix CI/CD workflow trigger from push: main to release: published — previously every push to main would attempt a PyPI publish
• Replace deprecated python setup.py sdist bdist_wheel with python -m build