Use strings as a non-terminal symbol in all grammars

This makes it easier to write the same logic for all Python versions

parso/python/tree.py

@@ -262,6 +262,15 @@ class String(Literal):
         return match.group(2)[:-len(match.group(1))]
 
 
+class FStringString(Leaf):
+    """
+    f-strings contain f-string expressions and normal python strings. These are
+    the string parts of f-strings.
+    """
+    type = 'fstring_string'
+    __slots__ = ()
+
+
 class _StringComparisonMixin(object):
     def __eq__(self, other):
         """