Message 80995 - Python tracker

Here is an updated patch against trunk.
To be honest I don't think it has a lot of future:
- it changes the lnotab format (to allow negative offsets)
- it rewrites the buggy block reordering code in the pure Python
compiler package, but nobody seems interested in maintaining that
package (see #2472).
Perhaps we should just declare the experiment failed and switch to
something else.