Fix incomplete initializer lists, unused vars, and some type warnings

[guitargeek]

Also fix the typos found with:
https://github.com/crate-ci/typos

Validated by the ROOT sync PR:
root-project/root#14507

[wlav]

I wasn't aware of the existence of the tp_vectorcall field ... I wrote that code for the py3.8 sources, where it isn't advertised (even though available). Currently, all CPPOverload objects have an fVectorCall field (as was per the documentation at the time). This is always set to mp_vectorcall. It'd be nice to remove that and safe a bit of memory.