backport of r60575 (issue #1750076): Debugger did not step on every iteration of a while statement. The mapping between bytecode offsets and source lines (lnotab) did not contain an entry for the beginning of the loop. Now it does, and the lnotab can be a bit larger: in particular, several statements on the same line generate several entries. However, this does not bother the settrace function, which will trigger only one 'line' event. The lnotab seems to be exactly the same as with python2.4.