Add Homebrew dependency on Python 'Cheetah' module

[rxseger]

This manual step is needed to install the Cheetah egg from Python:

pip install Cheetah

because I can't figure out how to add it as a dependency in the Homebrew formula, so it would be automatically installed.

There is a formulae repo for Python: https://github.com/homebrew/homebrew-python - and I use it for: numpy, scipy, and matplotlib, but they don't have cheetah (and might not want to, they say it is for "Python libraries with external dependencies that pip doesn't know how to handle"). Even if Cheetah has to be installed using pip, would be nice to remove the need to manually run it.

[rxseger]

https://github.com/Homebrew/brew/blob/master/share/doc/homebrew/Formula-Cookbook.md#specifying-gems-python-modules-go-projects-etc-as-dependencies

[rxseger]

Interestingly, pybombs gr-recipes removed the pip dependency and installs from git: gnuradio/gr-recipes@6cbc82a