Update about_asserts.py

[codeNameMahi]

added assertIn test

[dstanek]

Instead of doing this and making a test for assertIn I think it would be better to either:

1. work assertIn into another part of a lesson to test something else (there is not explicit assertEqual just to test assertEqual)
2. create a lesson specifically for doing unittest stuff

[gregmalcolm]

Thanks for the PR!

Yeah, probably don't want to drag out about_asserts too much. Students are usually hungry to get with something more interesting at that stage in the koans. I like @dstanek's suggestion of rolling it into a later koan. Maybe find an existing koan that could use this kind of assert and modify that.

Also if you're up for the challenge, can you provide a Python3 flavor as well please?

Closing for now, but feel free to open another PR if you want to take this forwards