Use absolute imports for consistency

[richvdh]

I'm keen for consistency, and neutral on whether we should prefer relative or absolute imports.

I thought this was explicitly covered by https://github.com/matrix-org/synapse/blob/develop/docs/code_style.md, but it seems I thought wrong.

In the absence of our own rules, we generally defer to Google's style guide, which is generally comprehensive and well-reasoned. In this case it says:

Import each module using the full pathname location of the module.

[richvdh]

(and also:

Do not use relative names in imports.

)

[squahtx]

Is there a linter check we can use to enforce this?

A quick search found https://github.com/MarcoGorelli/absolufy-imports which will reformat your imports, and https://pypi.org/project/flake8-absolute-import/