It's not so much about tests being slow: some just get stuck and timeout
travis.
Skipped all tests taking about more than 0.2s to run on my laptop.
Fast testing takes about 8s instead of 24.
Unfortunately PQcancel blocks, so it's not better than PQgetResult.
It has been suggested to use PQreset in non-blocking way but this would give
the Python program the burden of handling a connection done but not configured
in an unexpected place.
Dropped cyclic import from modules to tests: they were only working
because a second copy of the package was found in the project dir.
Use relative import so that 2to3 can do a good conversion.
