Adding a __iter__ method to skia.Iter

See
python/cpython#128161
"Defining iterator in a separate class no longer works in 3.13"

We have iterator for SkTextBlob defined by SkTextBlob::Iter(textblob),
which is the c++/pybind11 equivalent of the same situation.
Following the suggestion:
python/cpython#128161 (comment)

Also see actions/runner-images#11241

Fixes kyamagu#295

src/skia/TextBlob.cpp

@@ -65,6 +65,10 @@ py::class_<SkTextBlob::Iter::Run>(iter, "Run")
 
 iter
     .def(py::init<const SkTextBlob&>())
+    .def("__iter__",
+        [] (SkTextBlob::Iter& it) {
+            return it;
+        })
     .def("__next__",
         [] (SkTextBlob::Iter& it) {
             SkTextBlob::Iter::Run run;