Catch StopIteration in AbstractTable._enumerate_items

This makes PEP 479 enabled Pythons (such as 3.7) work again.

Otherwise you get:

    RuntimeError: generator raised StopIteration

Fixes pypa/pipenv#2426

prettytoml/elements/abstracttable.py

@@ -19,7 +19,10 @@ def _enumerate_items(self):
         """
         non_metadata = self._enumerate_non_metadata_sub_elements()
         while True:
-            yield next(non_metadata), next(non_metadata)
+            try:
+                yield next(non_metadata), next(non_metadata)
+            except StopIteration:
+                return
 
     def items(self):
         for (key_i, key), (value_i, value) in self._enumerate_items():