Issue #22599: Enhance tokenize.open() to be able to call it during Python

Issue #22599: Enhance tokenize.open() to be able to call it during Python
finalization.

Before the module kept a reference to the builtins module, but the module
attributes are cleared during Python finalization. Instead, keep directly a
reference to the open() function.

This enhancement is not perfect, calling tokenize.open() can still fail if
called very late during Python finalization.  Usually, the function is called
by the linecache module which is called to display a traceback or emit a
warning.