Add pep8-naming to the linting toolchain

[lemonsaurus]

We've decided to use this plugin in our linting toolchain, which allows us to test things like variable name casing (no camelCase please!) and whether classes and methods use cls and self.

Implementation

Here are the errors it can catch:

code	sample message
N801	class names should use CapWords convention
N802	function name should be lowercase
N803	argument name should be lowercase
N804	first argument of a classmethod should be named 'cls'
N805	first argument of a method should be named 'self'
N806	variable in function should be lowercase
N807	function name should not start and end with '__'
N811	constant imported as non constant
N812	lowercase imported as non lowercase
N813	camelcase imported as lowercase
N814	camelcase imported as constant
N815	mixedCase variable in class scope
N816	mixedCase variable in global scope

We have not decided exactly which of these we should ignore, so it will be up to the person who handles this issue to discuss this with the core devs and come to a consensus on whether we should ignore some of these. It will probably be easier to see which ones here are not necessary after actually running a lint with this plugin on our current codebase.

The person implementing this will also need to fix all the linting errors that occur once this has been added to the toolchain.

Steps

• [ ] Add the plugin to the toolchain
• [ ] Run it and look through the errors that are created. Are any of them out of touch with our style guidelines, and should we ignore some of these rules? Discuss with the core developers if you think so.
• [ ] Fix all the linting problems.