Fix a bug in the builtin format function.
Fix a bug in the builtin format function.

Hasimir/brython,kikocorreoso/brython,jonathanverner/brython,Hasimir/brython,jonathanverner/brython,brython-dev/brython,Hasimir/brython,brython-dev/brython,brython-dev/brython,jonathanverner/brython,jonathanverner/brython,Hasimir/brython,kikocorreoso/brython,kikocorreoso/brython