Allow bootstrapping sphinx
    
    1. Introduce --without websupport
    
     Sphinx (build)requires sphinxcontrib-websupport but that
     (build)requires sphinx. This bcond can be used to bootstrap this.
    
    2. Introduce --without tests
    
     The tests bring a lot of additional dependencies that make
     bootstrapping harder. This allows to build without tests.
    
    3. Build the docs with self
    
     The documentation was built with 'python', thus importing sphinx
     from python2's sitelib. Sphinx was installed because of the above
     mentioned buildrequirement of sphinxcontrib-websupport.
     Now current sphinx from PWD is used, also python3 is used when
     available, otherwise python2. Never just python.