Looks like simply adding `=> :optional` doesn't work as-is:

```
Error: undefined method `xy' for nil:NilClass
```

From
```
install_pythondir=#{lib}/#{python.xy}/site-packages
```

I thought from my reading of `Library/Homebrew/python_helper.rb` that it defined things appropriately if the system python was in use when no homebrew python was available.  But it looks like making it optional means it's not available at all when homebrew python isn't installed?  I thought this got invoked for each formula indirectly via its inclusion and use in `formula.rb`, but maybe I'm just misunderstanding how the internals work here.  I'm also not fully sure what the policy is WRT hard dependencies on the homebrew python vs using the system python.  Happy to make use of homebrew python optional if that's required; just need to know how to going about doing that properly--not clear to me after looking at the other formulas using python.