Add Python 3.8 compatibility

Fixes mcmtroffaes#167

See https://bugs.python.org/issue32892
See python/cpython#9445

sphinxcontrib/bibtex/cache.py

@@ -149,13 +149,19 @@ def visit_Name(self, node):
     def visit_Set(self, node):
         return frozenset(self.visit(elt) for elt in node.elts)
 
-    def visit_Str(self, node):
-        return node.s
-
     # NameConstant is Python 3.4 only so do not insist on coverage
     def visit_NameConstant(self, node):  # pragma: no cover
         return node.value
 
+    # Constant is Python 3.6+ only so do not insist on coverage
+    # Since 3.8 Num, Str, Bytes, NameConstant and Ellipsis are just Constant
+    def visit_Constant(self, node):  # pragma: no cover
+        return node.value
+
+    # Not used on 3.8+
+    def visit_Str(self, node):  # pragma: no cover
+        return node.s
+
     def generic_visit(self, node):
         _raise_invalid_node(node)