Enabling additional linters

[Rob Browning]

Before the release, I decided to take a look at all the default pylint
linters that we didn't have enabled. I've now enabled all of the ones I
think we might want at the moment, and while a lot of the changes
required to accommodate them weren't very significant, some were bug
fixes.

So I'd like to merge all those patches before the release, and enable
the related linters, and of course we can handle any that cause
trouble (say cross-version/platform) later.

--
Rob Browning
rlb @defaultvalue.org and @debian.org
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