tests: add pyflakes test

setup: make sure we use the imported module This prevent pyflakes to complains about it (and work around any possible demand import magic. I'm not sure why we do this important dance at all, but I'm leaving it there in doubt.

topicmap: whitespace

topiccache: directly instantiate a topiccache object when reading There is no need to rely on a temporary monkey patching.

readme: fix instruction to enable topic We moved the source code in 85390446f8c1 but we messed up the readme update. We are fixing it here.

stack: whitespace

stack: remove unused import