Doc: do not rely on checked-out Sphinx toolchain from svn.python.org anymore

Doc: do not rely on checked-out Sphinx toolchain from svn.python.org anymore

Nowadays it is likely that people will have Sphinx installed,
and if not, they will know how to install it.

This also simplifies life a lot for distribution packagers,
who typically do not want the doc build process to connect
to external web resources.