Do not set CO_NESTED.

Nested scopes were made mandatory in Python 2.2 and couldn't be turned off in any version that followed. There's no reason to use CO_NESTED any more.

Also, since Python has a slightly more efficient algorithm for calling functions with only CO_OPTIMIZED, CO_NEWLOCALS, and CO_NOFREE set, nested functions that don't reference outside variables should be a little faster. The difference is negligible in most projects, though.