Add a sensible repr to RSAPublicNumbers

Fixes #1230
I was going by the python [docs] which mentioned "If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment)."

I took it to mean that calling `eval()` on a __repr__ should return an approximate equivalent of the object. But sure, I can change to `<>` and just assert the string if you prefer it that way.

[docs]: https://docs.python.org/2/reference/datamodel.html#object.__repr__