Adds set_name method of pybind11::capsule class

[oleksandr-pavlyk]

This calls PyCapsule_SetName on the underlying Python object.

Description

Definition of pybind11::capsule class was missing set_name method.

This is useful if using pybind11 to implement DLPack protocol which requires to rename consumed capsule.

Suggested changelog entry:

pybind11::capsule::set_name added to mutate the name of the capsule instance.

[Skylion007]

Okay, we just need to fix our custom destructors with PyCapsule_GetName() instead of assuming the name is null.

[rwgk]

Okay, we just need to fix our custom destructors with PyCapsule_GetName() instead of assuming the name is null.

I think it's too complicated, trying to establish a safe context for managing the lifetime of name. Maybe even impossible, because we don't name who owns the passed name (is it static, or system malloc, from a pool, etc.). To not turn this into something big, I'd just add the thin wrapper (as-is) with the suggested comment.

[rwgk]

Looks good to me but (3.9-dbg (deadsnakes) • Valgrind • x64) ...

terminate called after throwing an instance of 'pybind11::error_already_set'
  what():  RuntimeError: Unable to cast Python instance of type <class 'numpy.ndarray'> to C++ type 'Eigen::Ref<Eigen::Matrix<double, -1, -1, 0, -1, -1>, 0, Eigen::Stride<-1, -1> >'
Fatal Python error: Aborted

huh?
(just to say I don't see what's wrong; needs debugging)

[rwgk]

Thanks Oleksandr!

[Skylion007]

This is actually a better solution then what I was going to suggest (as we still throw errors for if we can't get the context or the destructor is nullptr). Regardless, we can consider fixing those in a separate PR.