Fix #674 by explicitly setting return type to float in Cython version of HoppingWindow.ranges()

[forsberg]

Description

Fixes #674

Based on #673, or Travis will fail. Will happily rebase onto master once #672 is fixed.

Reorganizes tox a bit, using --import-mode append when running py.test, as before, the compiled (Cython versions) of modules were not actually tested in the Cython stage on Travis - reason being that due to faust not using a src/ directory layout, unit tests will import faust from the faust/ directory in the git clone, rather than from the version installed in the tox virtualenv.

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