Add python string standard lib (#21656)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21656
ghimport-source-id: cc7d7f68e33e95a97f6274c50823138aa4bacabb
Differential Revision: D15769256
Pulled By: bwasti
fbshipit-source-id: 1af487446361d90d03dce004c3e2169a3e62667d
