Add example using 'with' statement and urlopen

[njneered]

This pull request adds a beginner-friendly Python example to demonstrate the use of the with statement with urllib.request.urlopen() for safely opening and reading a URL. This addresses Issue #423.

The example is located in: context-managers/urlopen-example/with_urlopen_example.py

It includes a simple script that fetches content from https://www.python.org and prints the first 200 bytes. The code also contains helpful comments for learners.