Support older Python versions

[garyd203]

Versions required:

• Python 2.7
• Python 3 up to v3.5

Work (I think):

• Setup test runs with those versions
• Use six where necessary

Note that this is likely to be bigger than it looks, because flying-circus is also Python v3.6+ (for the same reasons as us, mainly because f strings and type annotations are appealing)

[garyd203]

As mentioned above, this would require Flying Circus to also support older Python versions. For lack of any meaningful use case (people trying to deploy from Ubuntu 14.04 LTS??!?), let's drop this ticket.