Ugly. A pile of new xxxFlags() functions, to communicate to the parser

that 'yield' is a keyword.  This doesn't help test_generators at all!  I
don't know why not.  These things do work now (and didn't before this
patch):

1. "from __future__ import generators" now works in a native shell.

2. Similarly "python -i xxx.py" now has generators enabled in the
   shell if xxx.py had them enabled.

3. This program (which was my doctest proxy) works fine:

from __future__ import generators

source = """\
def f():
    yield 1
"""

exec compile(source, "", "single") in globals()
print type(f())

Lib/doctest.py

@@ -348,6 +348,8 @@ Test passed.
 # 0,9,7    9-Feb-2001
 #    string method conversion
 
+from __future__ import generators
+
 __version__ = 0, 9, 7
 
 import types