Remove six usage

[rerpha]

we are no longer using python 2.7 and something tells me we won't be going back.

we should remove six from our code!

acceptance criteria

• [x] any uses of six are removed from our codebase
• [x] six is removed from requirements.txt

[LowriJenkins]

Relies on genie-python for requirements so should be reviewed and merged before genie python:

• Inst Servers
• Reflectometry Server
• Device Emulator
• IOC Test framework
• GUI
• Epics Upgrade
• Db Checker
• Db Changes
• Instrument Checker

Genie Python

Has its own requirement file that contains six, doesn't need to be reviewed and merged before genie-python (could leave these to a later time):

• LSI Correlator
• Instrument Scripts (Might be worth leaving these for now)
• HLM PV IMPORT

[Chsudeepta]

One linked PR remains open as that module wasn't written by us