PEP 750 bug and spec fix (python#58)

* Bugfix: increment `interpolationsidx` in `template_new()`
* Spec fix: don't return empty strings from `templateiter_next()`
---------

Co-authored-by: Lysandros Nikolaou <[email protected]>

Author: davepeck

Objects/templateobject.c

@@ -22,10 +22,15 @@ templateiter_next(templateiterobject *self)
     PyObject *item;
     if (self->from_strings) {
         item = PyIter_Next(self->stringsiter);
+        self->from_strings = 0;
+        if (PyUnicode_GET_LENGTH(item) == 0) {
+            Py_SETREF(item, PyIter_Next(self->interpolationsiter));
+            self->from_strings = 1;
+        }
     } else {
         item = PyIter_Next(self->interpolationsiter);
+        self->from_strings = 1;
     }
-    self->from_strings = !self->from_strings;
     return item;
 }
 
@@ -133,7 +138,7 @@ template_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
             if (!last_was_str) {
                 PyTuple_SET_ITEM(strings, stringsidx++, &_Py_STR(empty));
             }
-            PyTuple_SET_ITEM(interpolations, interpolationsidx, Py_NewRef(item));
+            PyTuple_SET_ITEM(interpolations, interpolationsidx++, Py_NewRef(item));
             last_was_str = 0;
         }
     }