Remove pre-Python 3.6 style code

[ewels]

The MultiQC code base has a bunch of stuff written for Python 2 that we no longer need. We should modernise this for a cleaner, more readable, better code strucutre.

Examples, but not limited to:

• Using OrderedDicts everywhere when we don't need to
• __future__ statements
• encode and other pre-unicode hacks
• Format strings (run flynt)
• ...

[ewels]

One crept in with a new module:

https://github.com/ewels/MultiQC/blob/9d00f14a317a3eb40c8edb5b4e92fde4bf11689e/multiqc/modules/truvari/truvari.py#L3

Not sure if we can have this in code_checks.py or something? Just so that the old style doesn't creep back in from PRs.

[vladsavelyev]

I think those creeping back shouldn't happen often, and we can handle that manually when reviewing the old PRs. If it becomes a problem, I'd add a check.