Change guidance: add site-packages to Python path with site.addsitedir

Address #30844 by changing our guidance about Python site-packages. Right now, we tell users to write this .pth file to pick up the Homebrew site-packages directory with a foreign or system Python:

> /usr/local/lib/python2.7/site-packages

But we should be telling them to use:

> import site; site.addsitedir("/usr/local/lib/python2.7/site-packages")

which requests that Python follow .pth files in our site-packages. This is necessary to support wxpython and perhaps other formulæ and may help enable less fragile Python installation strategies.

This PR changes the python_caveats and also adds a doctor check. The doctor check will probably annoy people. :) I'd like to encourage existing installations to migrate because I have secret plans that need .pth files to work, but maybe I should wait until we need it to add the doctor check.
Also, would be good to rephrase this warning to be something that indicates that if people don't care about Python development they can ignore it.