Homebrew Cython 0.20 (rebased onto dev_4_4)


This is the same as gh-3067 but rebased onto dev_4_4.

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This is a temporary PR to allow the Homebrew job to pass and review Formula changes while waiting for Pytables/Cython to release mutually compatible packages.
Second commit follows a comment by @kennethgillen about the usability of the Python dependencies installation script outside the context of Homebrew (and may be ported to `develop`).