New math functions in 3.5

[slozier]

These should be relatively simple to add:

• Two new constants have been added to the math module: inf and nan. (Contributed by Mark Dickinson in bpo-23185.)
• A new function isclose() provides a way to test for approximate equality. (Contributed by Chris Barker and Tal Einat in bpo-24270.)
• A new gcd() function has been added. The fractions.gcd() function is now deprecated. (Contributed by Mark Dickinson and Serhiy Storchaka in bpo-22486.)