`Literal` doesn't pass through to `ThingClient`.

[rwb27]

Describe the bug
An action argument or property typed as, for example, Literal["value1", "value2"] does generate a DataSchema typed as string, with an enum property that lists the values. However, this is currently ignored by #89, and the argument/property will just be typed as str, which is correct but less helpful.

Note that this is only relevant once #89 is merged.

To Reproduce
Create a Thing with a property typed as Literal["a", "b"], then generate a client for it (for example using ThingClient.from_url()) and check the type annotation.

Expected behaviour
The literal type should be reproduced on the client.

System:

• OS: [e.g. Ubuntu 19.10]
• Python version [e.g. 3.8]
• Version [e.g. 0.1.0]

Additional context
Add any other context about the problem here.

[rwb27]

Following up from #188, I've just checked that

from labthings_fastapi.thing_description import type_to_dataschema

type_to_dataschema(Literal["a", "b"]) # works

 type_to_dataschema(dict[Literal["a", "b"], int])
# Error while constructing DataSchema from the following dictionary:
...