chore: add err guard to capsule destructor and add a move to iostream (#3958)

* Add err guard to capsule destructor

* only uses ostream currently

* can these be noexcept

* Add back header

* fix for older compilers

* This should at least be noexcept

* Add missing move

* Apparently not noexcept for old llvm

Author: Skylion007

include/pybind11/iostream.h

@@ -100,7 +100,7 @@ class pythonbuf : public std::streambuf {
 
             if (size > remainder) {
                 str line(pbase(), size - remainder);
-                pywrite(line);
+                pywrite(std::move(line));
                 pyflush();
             }
 

include/pybind11/pytypes.h

@@ -1581,6 +1581,8 @@ class capsule : public object {
 
     capsule(const void *value, void (*destructor)(void *)) {
         m_ptr = PyCapsule_New(const_cast<void *>(value), nullptr, [](PyObject *o) {
+            // guard if destructor called while err indicator is set
+            error_scope error_guard;
             auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
             if (destructor == nullptr) {
                 if (PyErr_Occurred()) {