fix code for py37

src/_pytest/assertion/rewrite.py

@@ -687,12 +687,13 @@ class AssertionRewriter(ast.NodeVisitor):
                 expect_docstring
                 and isinstance(item, ast.Expr)
                 and isinstance(item.value, astStr)
-                and isinstance(item.value.value, str)
             ):
                 if sys.version_info >= (3, 8):
                     doc = item.value.value
                 else:
                     doc = item.value.s
+                if not isinstance(doc, str):
+                    return
                 if self.is_rewrite_disabled(doc):
                     return
                 expect_docstring = False