Fix: essentia on apple silicon

Fixes #30, fixes #32, fixes #33

The misconception in the linked issue description is that depends_on "numpy" should install numpy bindings, which essentia requires. However, homebrew's current numpy no longer supports python3.9, which is why the build was failing to find numpy. That is also why running python3.9 -m pip install numpy will allow essentia to build properly, although it is generally not recommended to install python modules to a homebrew python distribution manually.

Instead, I've done the following:

brew extract --version=0.29.30 libcython mtg/essentia
- get the last version of libcython (from homebrew) that uses python3.9
- restrict it to python3.9
brew extract --version=1.23.3 numpy mtg/essentia
- get the last version of numpy (from homebrew) that uses python3.9
- restrict it to python3.9, and use this tap's libcython
- add an explanation for users that this and homebrew-core/numpy conflict
update this formula to use an explicit python version, fixing that issue about symlinking python3.9 -> python manually

Note that the formulae i've extracted from homebrew-core are just the most recent versions that had depends_on "python@3.9". Newer versions of libcython and numpy support 3.9 but I didn't want to extract the current version and have to backport it in case there were other changes.

slight note: i think that --with-gaia bindings might be broken/not fully working, since that formula uses python@3.8. I can submit a pr later that will update that.

MTG / homebrew-essentia

Fix: essentia on apple silicon #36