Support UUID=True

[tdamsma]

Follow up of #95

SQLAlchemy allows setting UUID columns to either a string representation
of a UUID (e.g '46260785-9b7e-4a59-824f-af994a510673') or to a Python
uuid.UUID object.

Fixes #94

Includes #179 as the build is broken without it

Unfortunately we need support for typing.Literal which is only available for python >= 3.8, so with older versions nothing has changed.

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