Correct construct nan

Author: dflook

src/python_minifier/transforms/constant_folding.py

@@ -33,7 +33,7 @@ def visit_BinOp(self, node):
         except Exception as e:
             return node
 
-        if math.isnan(value):
+        if isinstance(value, float) and math.isnan(value):
             # There is no nan literal.
             new_node = ast.Call(func=ast.Name('float', ctx=ast.Load()), args=[ast.Str('nan')], keywords=[])
         elif isinstance(value, str):