SConstruct : Don't run Sphinx via `python`
SConstruct : Don't run Sphinx via `python`

On my system, `python` points at Python 3, but `sphinx-build` is installed for Python 2, and starts with `#!/usr/bin/python2`. Assume that Sphinx knows best.

johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,GafferHQ/gaffer,hradec/gaffer