Remove noexcept flag from api<T>::inc_ref/dec_ref()

That's because they call ``operator handle()``, which may raise an
exception.

docs/api_core.rst

@@ -72,11 +72,11 @@ following mixin class that lives in the ``nanobind::detail`` namespace.
 
       Obtain a const reference to the derived class.
 
-   .. cpp:function:: handle inc_ref() const noexcept
+   .. cpp:function:: handle inc_ref() const
 
       Increases the reference count and returns a reference to the Python object.
 
-   .. cpp:function:: handle ref_ref() const noexcept
+   .. cpp:function:: handle ref_ref() const
 
       Decreases the reference count and returns a reference to the Python object.
 
@@ -354,6 +354,16 @@ Without reference counting
       Check if the handle refers to a valid Python object. Equivalent to
       :cpp:func:`detail::api::is_valid()`
 
+   .. cpp:function:: handle inc_ref() const noexcept
+
+      Increases the reference count and returns a reference to the Python object.
+      Never raises an exception.
+
+   .. cpp:function:: handle ref_ref() const noexcept
+
+      Decreases the reference count and returns a reference to the Python object.
+      Never raises an exception.
+
    .. cpp:function:: PyObject * ptr() const
 
       Return the underlying ``PyObject*`` pointer.

include/nanobind/nb_types.h

@@ -105,8 +105,8 @@ template <typename Derived> class api : public api_tag {
     NB_INLINE bool is_none() const { return derived().ptr() == Py_None; }
     NB_INLINE bool is_type() const { return PyType_Check(derived().ptr()); }
     NB_INLINE bool is_valid() const { return derived().ptr() != nullptr; }
-    NB_INLINE handle inc_ref() const & noexcept;
-    NB_INLINE handle dec_ref() const & noexcept;
+    NB_INLINE handle inc_ref() const &;
+    NB_INLINE handle dec_ref() const &;
     iterator begin() const;
     iterator end() const;
 
@@ -680,11 +680,14 @@ template <typename Derived> NB_INLINE handle api<Derived>::type() const {
     return (PyObject *) Py_TYPE(derived().ptr());
 }
 
-template <typename Derived>
-NB_INLINE handle api<Derived>::inc_ref() const &noexcept {
+template <typename Derived> NB_INLINE handle api<Derived>::inc_ref() const & {
     return operator handle().inc_ref();
 }
 
+template <typename Derived> NB_INLINE handle api<Derived>::dec_ref() const & {
+    return operator handle().dec_ref();
+}
+
 template <typename Derived>
 NB_INLINE bool api<Derived>::is(handle value) const {
     return derived().ptr() == value.ptr();