Add conditional import for pickle5 in unit 2 hands-on

[bodoque007]

This PR resolves installation failures when setting up the Unit 2 hands-on exercise requirements. The current pickle5 dependency causes build errors on Python 3.8+ since the package is no longer needed for modern Python versions.

pickle5 is no longer required for Python >= 3.8 and Google Colab already runs Python 3.10+.
Yet, in case a student decides to do the hands-on exercise locally instead of doing it in Colab and happens to have a Python version older than 3.8, the code works just as fine.

Approach:

• Uses conditional requirement: pickle5; python_version < "3.8"
• Implements graceful fallback import pattern
• Ensures code works on both Colab and local environments regardless of Python version