Remove the horrid generators hack from doctest.py. This relies on a

Remove the horrid generators hack from doctest.py.  This relies on a
somewhat less horrid hack <wink>:  if a module does
    from __future__ import X
then the module dict D is left in a state such that (viewing X as a
string)
    D[X] is getattr(__future__, X)
So by examining D for all the names of future features, and making that
test for each, we can make a darned good guess as to which future-features
were imported by the module.  The appropriate flags are then sucked out
of the __future__ module, and passed on to compile()'s new optional
arguments (PEP 264).

Also gave doctest a meaningful __all__, removed the history of changes
(CVS serves that purpose now), and removed the __version__ vrbl (similarly;
before CVS, it was a reasonable clue, but not anymore).