chore: add err guard to capsule destructor and add a move to iostream

[Skylion007]

Description

• One fixes a potential bug in pycapsule destructor by adding an error_guard to one of the dtors.

Suggested changelog entry:

* Avoid potential bug in pycapsule destructor by adding an error_guard to one of the dtors

[rwgk]

I didn't get a chance to look closely today.

I don't fully understand how this plays with the pybind11_fail, but I feel sufficiently confident that this change will only make things better.

It would be good to note somewhere (here or PR description) how you came to add the error_guard (something failed? or did you just happen to notice?)

[Skylion007]

@rwgk I happened to notice that here:

pybind11/include/pybind11/pytypes.h

Line 1595 in a27914a

if (ptr == nullptr) {

an error_already_set could be raised for the wrong reason so we should probably enclose the whole thing in an err_scope. I may refactor this a bit more because we already create an err_scope for get_name which is now not necessary. Also, I am wondering if we instead of raising that error, should we just write unraisable like we do in the name_scope.

Thoughts @rwgk ?