Rely on .is_unbounded rather than comparing to S.(Negative)Infinity

Fixes a pypy failure:

dirac:~/git/sympy  $ cat my/pypy.py
from sympy import Float, S

print Float('+inf') in (S.Infinity,)
dirac:~/git/sympy  $ python my/pypy.py
True
dirac:~/git/sympy  $ pypy my/pypy.py
False