Make test pass with PyPy

Turns out that object() has a long repr() in PyPy.  This means that
the 'short objects' test was testing the wrong thing on PyPy, and
thus failing spuriously.

Fix by adding a `CustomRepr` object that deliberately creates a
short `__repr__`.
Add NEWS entry for pypy fix.
The ContentType repr() incorrectly used , instead of ;.