Issue #11074: Make 'tokenize' so it can be reloaded.

Issue #11074: Make 'tokenize' so it can be reloaded.

The module stored away the 'open' object as found in the global namespace
(which fell through to the built-in namespace) since it defined its own 'open'.
Problem is that if you reloaded the module it then grabbed the 'open' defined
in the previous load, leading to code that infinite recursed. Switched to
simply call builtins.open directly.