Various Python Fixes (and one Ruby)

- recommend `homebrew.pth` file instead of `PYTHONPATH`
- add PythonUtils class to simplify common use-cases e.g. iterating over python/python3
- cleanup PythonDependency and make it more strict
- add `mkvtoolnix` requirement for Ruby19

Question for @jacknagel: what's the right way to check inside a `Requirement` if `ARGV.build_bottle?` has been requested? Need this to always build bindings against the Homebrew Python (see discussion in #27112).