Finalize 'mypy synapse/'

[ShadowJonathan]

A cumulative PR with the final goal to be able to mypy synapse with current validation settings.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

Notes:

• todo depr-35 items are placed to signal actions needing to be taken when python 3.5 is deprecated (Deprecate python 3.5 #8347)
• type: ignore items should eventually be eliminated, or given a very good reason as to why they exist

PR To-Do list

• Find and add all correct DictProperty type annotations in synapse.events.EventBase

Signed-off-by: Jonathan de Jong <[email protected]>

[clokep]

@ShadowJonathan I'd suggest splitting this into separate PRs. It is much more likely to be reviewed and merged quickly if it is broken out!

[ShadowJonathan]

I'm splitting this into some other PRs, handling a few folders at a time