Without conflicting with #86.

This is almost entirely trivial changes:

- Make dummy methods that should never be called either raise
  NotImplementedError or call self.fail() in tests.
- Standardize the no-coverage pragma
- Add a few pragmas where coverage varies across
  versions/implementations, mostly for the sake of tox (travis should
  hit them all).

This bug was a reason (?) for a gratuitous fork of the _verify
function in z3c.testing.verify.

 '__code__', '__defaults__', '__func__', and '__self__' all work fine on
Python 2.6+.