Update oop_intro lecture content and TOC

Author: Zahra-khanzadeh

lectures/_toc.yml

@@ -9,7 +9,7 @@ parts:
   - file: python_by_example
   - file: functions
   # - file: python_essentials
-  # - file: oop_intro
+  - file: oop_intro
   # - file: names
   # - file: python_oop
   # - file: workspace