Allow disabling keepalive inference for constructors

[auscompgeek]

Sometimes we don't actually need keepalives for references passed to constructors. For example, none of Rotation3d's const-ref-taking constructor overloads hold the references in the instance.

Allow an explicit empty list to disable the constructor reference keepalive inference.

[virtuald]

Seems fine, but please add a test to test_ft_keepalive? And add a doc note.

[auscompgeek]

I actually expected it to work this way already when I read the existing doc.

keepalive: Optional[List[Tuple[int, int]]] = None

Adds py::keep_alive<x,y> to the function. Overrides automatic keepalive support, which retains references passed to constructors.