sc2: Removing unused dependency in requirements.txt

[MatthewMarinets]

What is this fixing or adding?

Removing an unused six dependency identified in #ap-core-dev a few minutes ago. Seems it's just been there unused since time immemorial.

How was this tested?

Did a pip uninstall six, and:

• Ran unit tests
• Generated
• Started the client and connected to a locally-hosted server
• Started a mission

If this makes graphical changes, please attach screenshots.

None

[MatthewMarinets]

Pinging @Ziktofel to request review

[black-sliver]

Can you fix the missing newline at end of the file while you are at it, so next time it won't have to change an extra line?