Investigate if T can be imported per file that uses it instead of via utils

[abbiemery]

T was moved into utils and imported from there to prevent sphinx from importing it for every single file for the docs. It would be preferable to have T = TypeVar("T") defined in each file that uses it as this is a common use case. We need to investigate with sphinx and see if we can resolve it.

[coretl]

In #700 we now make it a public member of a private module, and use it within that module, then sphinx ignore it. The only exception are public things like SignalDatatypeT which have a non-obvious bound