Get synapse/ to pass mypy

[callahad]

Our mypy configuration currently includes an explicit enumeration of files to check. That list covers 409 of the 491 Python files in the synapse/ directory.

We should:

• Get as many of those to pass as possible
• Invert the selection criteria so that all files in synapse/ are checked by default, while failing files are explicitly excluded.

As of 4249082, the omitted files are:

• storage/databases/__init__.py
• storage/databases/main/cache.py
• storage/schema/* (25 files)

We should probably start with switching from inclusion to exclusion, and burn down from there. Note that the next release of mypy will have significantly better syntax for multiple exclusions, but it's not out yet, so we'll have to get by with gnarlier regex-based syntax.

(Edited 2022-08-30 by dmr to reflect the status quo.)

[DMRobertson]

Things are much better now. The database modules remain a sticking point (related: #11733). It is very hard to untangle the MRO sphagetti into a form that mypy approves of and some extra thought is required.

The schema files are probably the next best thing to tackle. For bonus points, it'd be great if we could define a Protocol which specifies the interface expected of a schema upgrade file and have the mypy plugin enforce that.

[clokep]

The schema files are probably the next best thing to tackle. For bonus points, it'd be great if we could define a Protocol which specifies the interface expected of a schema upgrade file and have the mypy plugin enforce that.

I had some issues even getting mypy to see those files properly since they're not in modules (there are not __init__ files). So I'm unsure there's much we can do there?