Not sure why mypy is flagging these up now? Could be a mypy bug

```
synapse/__init__.py:67: error: Argument 1 to "set_json_library" has incompatible type Module; expected "JsonLibrary"  [arg-type]
synapse/__init__.py:67: note: Following member(s) of "Module json" have conflicts:
synapse/__init__.py:67: note:     JSONEncoder: expected "Encoder", got "Type[JSONEncoder]"

synapse/storage/engines/postgres.py:38: error: Argument 1 to "__init__" of "BaseDatabaseEngine" has incompatible type Module; expected "DBAPI2Module"  [arg-type]
synapse/storage/engines/postgres.py:38: note: Following member(s) of "Module psycopg2" have conflicts:
synapse/storage/engines/postgres.py:38: note:     DataError: expected "Type[Exception]", got "Type[DataError]"
synapse/storage/engines/postgres.py:38: note:     DatabaseError: expected "Type[Exception]", got "Type[DatabaseError]"
synapse/storage/engines/postgres.py:38: note:     <9 more conflict(s) not shown>
synapse/__init__.py:67: error: Argument 1 to "set_json_library" has incompatible type Module; expected "JsonLibrary"  [arg-type]

synapse/storage/engines/sqlite.py:29: error: Argument 1 to "__init__" of "BaseDatabaseEngine" has incompatible type Module; expected "DBAPI2Module"  [arg-type]
synapse/storage/engines/sqlite.py:29: note: Following member(s) of "Module sqlite3" have conflicts:
synapse/storage/engines/sqlite.py:29: note:     DataError: expected "Type[Exception]", got "Type[DataError]"
synapse/storage/engines/sqlite.py:29: note:     DatabaseError: expected "Type[Exception]", got "Type[DatabaseError]"
synapse/storage/engines/sqlite.py:29: note:     <9 more conflict(s) not shown>
```

These used to suppress

```
synapse/storage/engines/__init__.py:28: error: "__new__" must return a
class instance (got "NoReturn")  [misc]
```

and

```
synapse/http/matrixfederationclient.py:1270: error: "BaseException" has no attribute "reasons"  [attr-defined]
```

(note that we check `hasattr(e, "reasons")` above)

E.g.

```
tests/handlers/test_register.py:58: error: Missing return statement  [empty-body]
tests/handlers/test_register.py:108: error: Missing return statement  [empty-body]
```

Complaint was

```
synapse/logging/opentracing.py:450: error: Function "Type[Config]" could always be true in boolean context  [truthy-function]
```

Oops!

```
tests/rest/client/test_third_party_rules.py:428: error: Function "Callable[[], bool]" could always be true in boolean context  [truthy-function]
```

````
synapse/logging/opentracing.py:971: error: Argument 2 to "_custom_sync_async_decorator" has incompatible type "Callable[[Arg(Callable[P, R], 'func'), **P], _GeneratorContextManager[None]]"; expected "Callable[[Callable[P, R], **P], _GeneratorContextManager[None]]"  [arg-type]
synapse/logging/opentracing.py:1017: error: Argument 2 to "_custom_sync_async_decorator" has incompatible type "Callable[[Arg(Callable[P, R], 'func'), **P], _GeneratorContextManager[None]]"; expected "Callable[[Callable[P, R], **P], _GeneratorContextManager[None]]"  [arg-type]
````

See Shoobx/mypy-zope#91

```
tests/http/test_proxyagent.py:626: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:762: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:826: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:838: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:845: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:151: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:452: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:60: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:93: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:127: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:152: error: Statement is unreachable  [unreachable]
```

I had to add two new linter ignores here, but it did detect an
incomplete subclass in the tests.

Some extra context in:
- matrix-org/python-canonicaljson#57
- python/mypy#6002
- https://mypy.readthedocs.io/en/latest/common_issues.html#covariant-subtyping-of-mutable-protocol-members-is-rejected

so the protocol check just works

I tried to workaround the ignores but found it too much trouble.

I think the corresponding issue is
python/mypy#12909. The mypy repo has a PR
claiming to fix this (python/mypy#14677) which
might mean this gets resolved soon?