Get synapse/ to pass mypy

[matrixbot]

This issue has been migrated from #11271.

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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.)