mypy issues with non-async resolvers

As found in PR #94, it seems that there are some type errors present:

```
src/coniql/strawberry_schema.py:145: error: Incompatible types in assignment (expression has type "coniql.types.ChannelValue | None", variable has type "coniql.strawberry_schema.ChannelValue | None") [assignment]
src/coniql/strawberry_schema.py:147: error: Incompatible types in assignment (expression has type "coniql.types.ChannelTime | None", variable has type "coniql.strawberry_schema.ChannelTime | None") [assignment]
src/coniql/strawberry_schema.py:149: error: Incompatible types in assignment (expression has type "coniql.types.ChannelStatus | None", variable has type "coniql.strawberry_schema.ChannelStatus | None") [assignment]
```

These seemed to have been caused by an update to Strawberry, [in version 0.186.0](https://github.com/strawberry-graphql/strawberry/releases/tag/0.186.0), where parts of the MyPy plugin were removed. For reasons unknown, having the resolvers as `async` functions seemed to mask the error they found.

I believe these errors are valid - we are indeed returning a `types.ChannelValue` when we specify it should be a `strawberry_schema.ChannelValue`. I don't know the exact mechanism that's running that enables this apparent conflict to successfully resolve itself - logically the two are differently structured and so should not be interchangeable.

This may also be a good time to do something about all the near-duplicate names we have - there's one set in `strawberry_schema.py` that mirror the names of types found in `types.py`. Conceptually this is to provide a mapping between the Strawberry and the Python representation of objects, but perhaps we should revist this (and/or just scrap the Python versions and do it all in the Strawberry classes, making use of [private fields](https://strawberry.rocks/docs/types/private)).