MyPy-lint the whole codebase

[ShadowJonathan]

This is a bit of a "sentinel" issue, to be able to mypy-check the whole of synapse/ and tests/, at the very least.

While working on #9682 i noticed that mypy is missing a lot of corners, so im making this issue to work towards.

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Simply; the goal is to reduce the [mypy] -> files= key in this file down to synapse/,, so that one mypy pass goes over the entire database unequivocally.

[clokep]

Is this essentially #8351?

[ShadowJonathan]

No, that issue calls for var: Type instead of var = [...] # type: Type, it also calls for more variable annotations.

What this issue is about is to make these lines effectively only be synapse/,, so to speak, to have mypy check the entire synapse/ codebase instead of a subset of subdirectories and files.

[clokep]

Closing in favor of #11271.