[Anuragluck]

Fixes #5423

ping_url() imported urlib.request instead of urllib.request, then fell back to the obsolete urllib2 module. On Python 3, this caused ModuleNotFoundError before the URL could be checked.

This PR imports urlopen from urllib.request, removes the Python 2 fallback, and adds tests for both successful and failed URL checks.

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled and links issue ping_url() in commoncode/fetch.py raises ImportError on Python 3 (typo in import) #5423
• Added focused unit tests; 2 tests pass locally
• Commit is on a dedicated feature branch
• Updated CHANGELOG.rst
• Updated AUTHORS.rst