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2.7.3 is a kind of bridge between the two and most people still using older version, the transition will take some time.
If you try to use 3 in the course you will run into problems.
The biggest reason is that the MIT course materials are written for 2.6 and the Codecademy materials to 2.7. We're not creating any new materials for the course, so we are going with what the legacy materials use.
They made a lot of major changes in version 3. If you watch the lecture, you'll see the professor doesn't seem to think they are improvements. Since 2.x is widely used and the purpose of the class is to learn programming skills it seems like a logical choice to stick with it.
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More simple and less complex? New versions usually add in more features and can have a longer learning curve. Older software versions do not require additional learning as some are already accustomed to such versions.