Closes sphinx-doc#5076: [ext/napoleon] explicitly catch StopIteration

Per PEP 479, Python 3.7 no longer allows bubbling up StopIteration
outside of a generator. Instead, wrap attribute parsing in a try
block and provide a sane default in case it raises an exception
([]).

sphinx/ext/napoleon/docstring.py

@@ -527,7 +527,14 @@ def _parse(self):
         self._parsed_lines = self._consume_empty()
 
         if self._name and (self._what == 'attribute' or self._what == 'data'):
-            self._parsed_lines.extend(self._parse_attribute_docstring())
+            # Implicit stop using StopIteration no longer allowed in
+            # Python 3.7; see PEP 479
+            res = []
+            try:
+                res = self._parse_attribute_docstring()
+            except StopIteration as e:
+                pass
+            self._parsed_lines.extend(res)
             return
 
         while self._line_iter.has_next():