caveats: recommend Python .pth file instead.

This is better than PYTHONPATH as it doesn't mess with incompatible
versions of Python.

Closes #27138.
Add PythonUtils class.
python_dependency: fixes, features, cleanup.

- PythonDependency now implies Python 2.7
- PythonDependency now uses brewed Python for bottling
- Use double-quotes everywhere

Closes #27112.
libxml2: use :python on 10.6.
pyqt: use PythonUtils.
pyqt5: use PythonUtils.
sip: use PythonUtils.
pygobject3: use PythonUtils.
qscintilla2: use PythonUtils.
v8: just use PythonDependency as it implies 2.7.
requirements: remove unneeded Python27Dependency.

PythonDependency now implies this.
boost: use new python requirement bottle logic.
mkvtoolnix: check for Ruby 1.9.
WIP