std::shared_ptr<T> bindings: avoid leaks at interpreter shutdown

The ``nb::shared_ptr<T>`` binding implementation may sometimes need to
create a shared pointer wrapper around an object that was originally
constructed in Python. Eventual expiry of this shared pointer will
decrease the reference count of the underlying Python object. However,
this is unsafe to do when Python is already shut down, which the
implementation previously checked by querying ``Py_IsInitialized()``.

It turns out that this is a bit too conservative since it also prevents
proper cleanup *during* shutdown time/finalization
(``Py_IsInitialized()`` already returns zero at this point). This commit
changes the criterion to ``nb::is_alive()``.

include/nanobind/stl/shared_ptr.h

@@ -19,7 +19,7 @@ NAMESPACE_BEGIN(detail)
 struct py_deleter {
     void operator()(void *) noexcept {
         // Don't run the deleter if the interpreter has been shut down
-        if (!Py_IsInitialized())
+        if (!is_alive())
             return;
         gil_scoped_acquire guard;
         Py_DECREF(o);